gh-110383: Improve accuracy of str.split() and str.rsplit() docstrings

[erlend-aasland]

Mention split direction in the docstring body,
instead of in the 'maxsplit' param docstring.

• Issue: Issues from docs@python.org #110383

[erlend-aasland]

Alternative to #111247 and #113353.

[hugovk]

Looks good:

❯ ./python.exe
Python 3.13.0a2+ (heads/splitter!-dirty:c50dd463dd, Dec 21 2023, 14:51:45) [Clang 15.0.0 (clang-1500.1.0.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> help(str().split)
Help on built-in function split:

split(sep=None, maxsplit=-1) method of builtins.str instance
    Return a list of the substrings in the string, using sep as the separator string.

      sep
        The separator used to split the string.

        When set to None (the default value), will split on any whitespace
        character (including \n \r \t \f and spaces) and will discard
        empty strings from the result.
      maxsplit
        Maximum number of splits.
        -1 (the default value) means no limit.

    Splitting starts at the front of the string and works to the end.

    Note, str.split() is mainly useful for data that has been intentionally
    delimited.  With natural text that includes punctuation, consider using
    the regular expression module.

>>> help(str().rsplit)
Help on built-in function rsplit:

rsplit(sep=None, maxsplit=-1) method of builtins.str instance
    Return a list of the substrings in the string, using sep as the separator string.

      sep
        The separator used to split the string.

        When set to None (the default value), will split on any whitespace
        character (including \n \r \t \f and spaces) and will discard
        empty strings from the result.
      maxsplit
        Maximum number of splits.
        -1 (the default value) means no limit.

    Splitting starts at the end of the string and works to the front.

>>>

Thanks everyone!

[bedevere-app]

GH-113379 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-113380 is a backport of this pull request to the 3.11 branch.