argparse: improve mutually exclusive groups

The current wording "Note that currently mutually exclusive argument
groups do not support the title and description arguments" implies
that in the future they will. However, that doesn't seem needed, as
one can simply wrap the mutually exclusive group into an argument with
with the desired title/description.

Doc/library/argparse.rst

@@ -1915,9 +1915,13 @@ Mutual exclusion
      usage: PROG [-h] (--foo | --bar)
      PROG: error: one of the arguments --foo --bar is required
 
-   Note that currently mutually exclusive argument groups do not support the
+   Note that mutually exclusive argument groups do not support the
    *title* and *description* arguments of
-   :meth:`~ArgumentParser.add_argument_group`.
+   :meth:`~ArgumentParser.add_argument_group`. If you want a mutually exclusive group
+   to be formatted separately, wrap it via an argument group:
+
+     >>> parser = argparse.ArgumentParser(prog='PROG')
+     >>> group = parser.add_argument_group('group').add_mutually_exclusive_group()
 
 
 Parser defaults