MNT: local changes to vendored docs script github_link.py

As far as I can see, we had just added a few lines to tell this vendored
script originates in scikit-learn.

File initially copied in #1258, and not changed since.

Author: DimitriPapadopoulos

docs/sphinxext/github_link.py

@@ -1,3 +1,7 @@
+"""
+This vendored script comes from scikit-learn:
+https://github.com/scikit-learn/scikit-learn/blob/master/doc/sphinxext/github_link.py
+"""
 import inspect
 import os
 import subprocess