bpo-37751: Update codecs.register() doc. (pythonGH-25643)

Author: methane

Doc/library/codecs.rst

@@ -159,9 +159,13 @@ function:
 .. function:: register(search_function)
 
  Register a codec search function. Search functions are expected to take one
- argument, being the encoding name in all lower case letters, and return a
- :class:`CodecInfo` object. In case a search function cannot find
- a given encoding, it should return ``None``.
+ argument, being the encoding name in all lower case letters with hyphens
+ and spaces converted to underscores, and return a :class:`CodecInfo` object.
+ In case a search function cannot find a given encoding, it should return
+ ``None``.
+
+ .. versionchanged:: 3.9
+ Hyphens and spaces are converted to underscore.
 
 
 .. function:: unregister(search_function)