gh-110131: Document how a non-empty default works with append and extend (GH-158155)

(cherry picked from commit 70bc048)

Co-authored-by: Dmitry Voropaev <144015565+v0ropaev@users.noreply.github.com>

Author: v0ropaev

Doc/library/argparse.rst

@@ -791,9 +791,9 @@ how the command-line arguments should be handled. The supplied actions are:
 
 * ``'append'`` - This appends each argument value to a list.
   It is useful for allowing an option to be specified multiple times.
-  If the default value is a non-empty list, the parsed value will start
-  with the default list's elements and any values from the command line
-  will be appended after those default values. Example usage::
+  If the default value is a non-empty list, the parsed value for the option
+  will start with the default list's elements and any values from the
+  command line will be appended after those default values. Example usage::
 
     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', action='append', default=['0'])
@@ -818,12 +818,16 @@ how the command-line arguments should be handled. The supplied actions are:
   value ``'+'`` or ``'*'``.
   Note that when nargs_ is ``None`` (the default) or ``'?'``, each
   character of the argument string will be appended to the list.
+  If the default value is a non-empty list, the parsed value for the option
+  will start with the default list's elements and any values from the
+  command line will be appended after those default values.
   Example usage::
 
     >>> parser = argparse.ArgumentParser()
-    >>> parser.add_argument("--foo", action="extend", nargs="+", type=str)
+    >>> parser.add_argument("--foo", action="extend", nargs="+", type=str,
+    ...                     default=["d1"])
     >>> parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"])
-    Namespace(foo=['f1', 'f2', 'f3', 'f4'])
+    Namespace(foo=['d1', 'f1', 'f2', 'f3', 'f4'])
 
   .. versionadded:: 3.8