Enhance os.scandir() doc

Issue #26248, patch written by Ben Hoyt:

1) Clarify that the return values of is_dir()/is_file()/etc are cached
   separately for follow_symlinks True and False.
2) Be more specific about when the functions require a system call, and how it
   relates to caching and follow_symlinks.
3) DRY up common stuff between is_dir and is_file by saying "Caching, system
   calls made, and exceptions raised are as per is_dir" in is_file.
4) Tweak to the first paragraph of docs for is_dir/is_file to simplify: assume
   the follow_symlinks=True default, then note the follow_symlinks=False
   non-default case after.

Doc/library/os.rst

@@ -1960,62 +1960,65 @@ features:
 
       Return the inode number of the entry.
 
-      The result is cached on the ``DirEntry`` object, use ``os.stat(entry.path,
+      The result is cached on the ``DirEntry`` object. Use ``os.stat(entry.path,
       follow_symlinks=False).st_ino`` to fetch up-to-date information.
 
-      On Unix, no system call is required.
+      On the first, uncached call, a system call is required on Windows but
+      not on Unix.
 
    .. method:: is_dir(\*, follow_symlinks=True)
 
-      If *follow_symlinks* is ``True`` (the default), return ``True`` if the
-      entry is a directory or a symbolic link pointing to a directory;
-      return ``False`` if it is or points to any other kind of file, or if it
-      doesn't exist anymore.
+      Return ``True`` if this entry is a directory or a symbolic link pointing
+      to a directory; return ``False`` if the entry is or points to any other
+      kind of file, or if it doesn't exist anymore.
 
       If *follow_symlinks* is ``False``, return ``True`` only if this entry
-      is a directory; return ``False`` if it is any other kind of file
-      or if it doesn't exist anymore.
+      is a directory (without following symlinks); return ``False`` if the
+      entry is any other kind of file or if it doesn't exist anymore.
+
+      The result is cached on the ``DirEntry`` object, with a separate cache
+      for *follow_symlinks* ``True`` and ``False``. Call :func:`os.stat` along
+      with :func:`stat.S_ISDIR` to fetch up-to-date information.
 
-      The result is cached on the ``DirEntry`` object. Call :func:`os.stat`
-      along with :func:`stat.S_ISDIR` to fetch up-to-date information.
+      On the first, uncached call, no system call is required in most cases.
+      Specifically, for non-symlinks, neither Windows or Unix require a system
+      call, except on certain Unix file systems, such as network file systems,
+      that return ``dirent.d_type == DT_UNKNOWN``. If the entry is a symlink,
+      a system call will be required to follow the symlink unless
+      *follow_symlinks* is ``False``.
 
       This method can raise :exc:`OSError`, such as :exc:`PermissionError`,
       but :exc:`FileNotFoundError` is caught and not raised.
 
-      In most cases, no system call is required.
-
    .. method:: is_file(\*, follow_symlinks=True)
 
-      If *follow_symlinks* is ``True`` (the default), return ``True`` if the
-      entry is a file or a symbolic link pointing to a file; return ``False``
-      if it is or points to a directory or other non-file entry, or if it
-      doesn't exist anymore.
+      Return ``True`` if this entry is a file or a symbolic link pointing to a
+      file; return ``False`` if the entry is or points to a directory or other
+      non-file entry, or if it doesn't exist anymore.
 
       If *follow_symlinks* is ``False``, return ``True`` only if this entry
-      is a file; return ``False`` if it is a directory or other non-file entry,
-      or if it doesn't exist anymore.
+      is a file (without following symlinks); return ``False`` if the entry is
+      a directory or other non-file entry, or if it doesn't exist anymore.
 
-      The result is cached on the ``DirEntry`` object. Call :func:`os.stat`
-      along with :func:`stat.S_ISREG` to fetch up-to-date information.
-
-      This method can raise :exc:`OSError`, such as :exc:`PermissionError`,
-      but :exc:`FileNotFoundError` is caught and not raised.
-
-      In most cases, no system call is required.
+      The result is cached on the ``DirEntry`` object. Caching, system calls
+      made, and exceptions raised are as per :func:`~DirEntry.is_dir`.
 
    .. method:: is_symlink()
 
       Return ``True`` if this entry is a symbolic link (even if broken);
-      return ``False`` if it points to a directory or any kind of file,
+      return ``False`` if the entry points to a directory or any kind of file,
       or if it doesn't exist anymore.
 
       The result is cached on the ``DirEntry`` object. Call
       :func:`os.path.islink` to fetch up-to-date information.
 
-      The method can raise :exc:`OSError`, such as :exc:`PermissionError`,
-      but :exc:`FileNotFoundError` is caught and not raised.
+      On the first, uncached call, no system call is required in most cases.
+      Specifically, neither Windows or Unix require a system call, except on
+      certain Unix file systems, such as network file systems, that return
+      ``dirent.d_type == DT_UNKNOWN``.
 
-      In most cases, no system call is required.
+      This method can raise :exc:`OSError`, such as :exc:`PermissionError`,
+      but :exc:`FileNotFoundError` is caught and not raised.
 
    .. method:: stat(\*, follow_symlinks=True)
 
@@ -2023,17 +2026,17 @@ features:
       follows symbolic links by default; to stat a symbolic link add the
       ``follow_symlinks=False`` argument.
 
-      On Unix, this method always requires a system call. On Windows,
-      ``DirEntry.stat()`` requires a system call only if the
-      entry is a symbolic link, and ``DirEntry.stat(follow_symlinks=False)``
-      never requires a system call.
+      On Unix, this method always requires a system call. On Windows, it
+      only requires a system call if *follow_symlinks* is ``True`` and the
+      entry is a symbolic link.
 
       On Windows, the ``st_ino``, ``st_dev`` and ``st_nlink`` attributes of the
       :class:`stat_result` are always set to zero. Call :func:`os.stat` to
       get these attributes.
 
-      The result is cached on the ``DirEntry`` object. Call :func:`os.stat`
-      to fetch up-to-date information.
+      The result is cached on the ``DirEntry`` object, with a separate cache
+      for *follow_symlinks* ``True`` and ``False``. Call :func:`os.stat` to
+      fetch up-to-date information.
 
    Note that there is a nice correspondence between several attributes
    and methods of ``DirEntry`` and of :class:`pathlib.Path`.  In