gh-110131: Document argparse's extend action with a non-empty default

The 'append' bullet says that a non-empty list default is kept and that
command-line values land after it.  'extend' works the same way for an
option, but the docs say nothing about it, which is the confusion
reported in gh-110131.

The sentence is scoped to the option deliberately.  For a positional with
nargs='*' or '?' and no command-line values, _get_values hands the default
back as the parsed value while the same default is already in the
namespace, so the action applies it twice and extend(default=['X'])
yields ['X', 'X'].  The unqualified wording would be wrong there.

Author: v0ropaev

Doc/library/argparse.rst

@@ -818,6 +818,9 @@ 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()