sphinx-autogen: Move parser to a separate function

For the same reasons as the 'sphinx-apidoc' move. We also take the
opportunity to add a help string and epilog while we're at it.

Signed-off-by: Stephen Finucane <stephen@that.guru>

sphinx/ext/autosummary/generate.py

@@ -60,16 +60,30 @@ if False:
     from sphinx.environment import BuildEnvironment  # NOQA
 
 
-def main(argv=sys.argv[1:]):
-    # type: (List[str]) -> None
+def get_parser():
+    # type: () -> argparse.ArgumentParser
     parser = argparse.ArgumentParser(
-        usage='%(prog)s [OPTIONS] <SOURCE_FILE>...')
+        usage='%(prog)s [OPTIONS] <SOURCE_FILE>...',
+        epilog='For more information, visit <http://sphinx-doc.org/>.',
+        description="""
+Generate ReStructuredText using autosummary directives.
+
+sphinx-autogen is a frontend to sphinx.ext.autosummary.generate. It generates
+the reStructuredText files from the autosummary directives contained in the
+given input files.
+
+The format of the autosummary directive is documented in the
+``sphinx.ext.autosummary`` Python module and can be read using::
+
+  pydoc sphinx.ext.autosummary
+""")
 
     parser.add_argument('--version', action='version', dest='show_version',
                         version='%%(prog)s %s' % __display_version__)
 
     parser.add_argument('source_file', nargs='+',
                         help='source files to generate rST files for')
+
     parser.add_argument('-o', '--output-dir', action='store',
                         dest='output_dir',
                         help='directory to place all output in')
@@ -86,7 +100,12 @@ def main(argv=sys.argv[1:]):
                         help='document imported members (default: '
                               '%(default)s)')
 
-    args = parser.parse_args(argv)
+    return parser
+
+
+def main(argv=sys.argv[1:]):
+    # type: (List[str]) -> None
+    args = get_parser().parse_args(argv)
     generate_autosummary_docs(args.source_file, args.output_dir,
                               '.' + args.suffix,
                               template_dir=args.templates,