Closes #11807: document argparse add_subparsers method better.

Patch by Filip Gruszczyński.

Doc/library/argparse.rst

@@ -1452,7 +1452,10 @@ Other utilities
 Sub-commands
 ^^^^^^^^^^^^
 
-.. method:: ArgumentParser.add_subparsers()
+.. method:: ArgumentParser.add_subparsers([title], [description], [prog], \
+                                          [parser_class], [action], \
+                                          [option_string], [dest], [help], \
+                                          [metavar])
 
    Many programs split up their functionality into a number of sub-commands,
    for example, the ``svn`` program can invoke sub-commands like ``svn
@@ -1466,6 +1469,30 @@ Sub-commands
    command name and any :class:`ArgumentParser` constructor arguments, and
    returns an :class:`ArgumentParser` object that can be modified as usual.
 
+   Description of parameters:
+
+   * title - title for the sub-parser group in help output; by default
+     "subcommands" if description is provided, otherwise uses title for
+     positional arguments
+
+   * description - description for the sub-parser group in help output, by
+     default None
+
+   * prog - usage information that will be displayed with sub-command help,
+     by default the name of the program and any positional arguments before the
+     subparser argument
+
+   * parser_class - class which will be used to create sub-parser instances, by
+     default the class of the current parser (e.g. ArgumentParser)
+
+   * dest - name of the attribute under which sub-command name will be
+     stored; by default None and no value is stored
+
+   * help - help for sub-parser group in help output, by default None
+
+   * metavar - string presenting available sub-commands in help; by default it
+     is None and presents sub-commands in form {cmd1, cmd2, ..}
+
    Some example usage::
 
      >>> # create the top-level parser