[3.12] gh-89819: Add argument_default and conflict_handler to add_argument_group() docs (GH-125379) (GH-125539)

(cherry picked from commit c9826c1)

Co-authored-by: Savannah Ostrowski <savannahostrowski@gmail.com>

Author: miss-islington

Doc/library/argparse.rst

@@ -1740,7 +1740,8 @@ FileType objects
 Argument groups
 ^^^^^^^^^^^^^^^
 
-.. method:: ArgumentParser.add_argument_group(title=None, description=None)
+.. method:: ArgumentParser.add_argument_group(title=None, description=None, *, \
+                                              [argument_default], [conflict_handler])
 
    By default, :class:`ArgumentParser` groups command-line arguments into
    "positional arguments" and "options" when displaying help
@@ -1785,6 +1786,11 @@ Argument groups
 
        --bar BAR  bar help
 
+   The optional, keyword-only parameters argument_default_ and conflict_handler_
+   allow for finer-grained control of the behavior of the argument group. These
+   parameters have the same meaning as in the :class:`ArgumentParser` constructor,
+   but apply specifically to the argument group rather than the entire parser.
+
    Note that any arguments not in your user-defined groups will end up back
    in the usual "positional arguments" and "optional arguments" sections.