Docstring for Repository.create_git_tag_and_release (#1425)

The function create_git_tag_and_release had no docstring, so it didn't
appear anywhere in our docs. Add a short one linking to the two methods
it calls.

Fixes #1402

github/Repository.py

@@ -961,6 +961,10 @@ class Repository(github.GithubObject.CompletableGithubObject):
         draft=False,
         prerelease=False,
     ):
+        """
+        Convenience function that calls :meth:`Repository.create_git_tag` and
+        :meth:`Repository.create_git_release`.
+        """
         self.create_git_tag(tag, tag_message, object, type, tagger)
         return self.create_git_release(
             tag,