bpo-37556 Extend help to include latest overrides (GH-14701)

Modify the help in cpython/PC/launcher.c to show users that "latest" can be overridden by shebang, PY_PYTHON[n] or py.ini files. Also show that script [args] is optional by enclosing in square brackets.

Automerge-Triggered-By: @zooba

Misc/NEWS.d/next/Windows/2019-07-11-06-11-09.bpo-37556.sygMUU.rst

@@ -0,0 +1 @@
+Extend py.exe help to mention overrides via venv, shebang, environmental variables & ini files.

PC/launcher.c

@@ -1520,7 +1520,7 @@ show_help_text(wchar_t ** argv)
 Python Launcher for Windows Version %ls\n\n", version_text);
     fwprintf(stdout, L"\
 usage:\n\
-%ls [launcher-args] [python-args] script [script-args]\n\n", argv[0]);
+%ls [launcher-args] [python-args] [script [script-args]]\n\n", argv[0]);
     fputws(L"\
 Launcher arguments:\n\n\
 -2     : Launch the latest Python 2.x version\n\
@@ -1536,6 +1536,15 @@ Launcher arguments:\n\n\
     }
     fputws(L"\n-0  --list       : List the available pythons", stdout);
     fputws(L"\n-0p --list-paths : List with paths", stdout);
+    fputws(L"\n\n If no script is specified the specified interpreter is opened.", stdout);
+    fputws(L"\nIf an exact version is not given, using the latest version can be overridden by", stdout);
+    fputws(L"\nany of the following, (in priority order):", stdout);
+    fputws(L"\n An active virtual environment", stdout);
+    fputws(L"\n A shebang line in the script (if present)", stdout);
+    fputws(L"\n With -2 or -3 flag a matching PY_PYTHON2 or PY_PYTHON3 Enviroment variable", stdout);
+    fputws(L"\n A PY_PYTHON Enviroment variable", stdout);
+    fputws(L"\n From [defaults] in py.ini in your %LOCALAPPDATA%\\py.ini", stdout);
+    fputws(L"\n From [defaults] in py.ini beside py.exe (use `where py` to locate)", stdout);
     fputws(L"\n\nThe following help text is from Python:\n\n", stdout);
     fflush(stdout);
 }