Add description for function do_list_extensions in cinderclient

Now the current cinderclient version default use v3 interface.
And the do_list_extensions of V3 inherited V2. The do_list_extensions
of V2 does not have __doc__.For the cli-reference, there is a specific
docs tool used  to auto-generate the cli-reference files.The docs tool
find the function __doc__ that starts with 'do_',as The
do_list_extensions's __doc__ is none,so the subcommand 'list-extensions'
no desc.

python-cinderclient/cinderclient/shell.py
---------------------------------
def _find_actions(self, subparsers, actions_module, version,
                      do_help, input_args):
        for attr in (a for a in dir(actions_module) if
a.startswith('do_')):
            # I prefer to be hyphen-separated instead of underscores.
            command = attr[3:].replace('_', '-')
            callback = getattr(actions_module, attr)
            desc = callback.__doc__ or ''
            .......

The OpenStack contributor guide has instructions. The guide has a
chapter on how to use the duto-generate docs tools
- https://docs.openstack.org/contributor-guide/doc-tools.html
Specifically, the chapter on the command line reference explains
how the tool works -
https://docs.openstack.org/contributor-guide/doc-tools/cli-reference.html

Change-Id: I5484d1e25b4ec11182b27c80fc43e816b1a12736
Closes-Bug: #1674934

cinderclient/v2/contrib/list_extensions.py

@@ -37,14 +37,10 @@ class ListExtManager(base.Manager):
         return self._list("/extensions", 'extensions')
 
 
-def list_extensions(client, _args):
+def do_list_extensions(client, _args):
     """
     Lists all available os-api extensions.
     """
     extensions = client.list_extensions.show_all()
     fields = ["Name", "Summary", "Alias", "Updated"]
     utils.print_list(extensions, fields)
-
-
-def do_list_extensions(client, _args):
-    return list_extensions(client, _args)