Issue #27075: Link to StreamReader and StreamWriter docs

Patch by Ville Skyttä.

Doc/library/codecs.rst

@@ -137,16 +137,16 @@ these additional functions which use :func:`lookup` for the codec lookup:
 
 .. function:: getreader(encoding)
 
-   Look up the codec for the given encoding and return its StreamReader class or
-   factory function.
+   Look up the codec for the given encoding and return its :class:`StreamReader`
+   class or factory function.
 
    Raises a :exc:`LookupError` in case the encoding cannot be found.
 
 
 .. function:: getwriter(encoding)
 
-   Look up the codec for the given encoding and return its StreamWriter class or
-   factory function.
+   Look up the codec for the given encoding and return its :class:`StreamWriter`
+   class or factory function.
 
    Raises a :exc:`LookupError` in case the encoding cannot be found.