bpo-14826: document that URLopener quotes fullurl. (GH-12758)

(cherry picked from commit 2fb2bc8)

Co-authored-by: Gregory P. Smith <[email protected]>

Author: miss-islington

Doc/library/urllib.request.rst

@@ -1435,6 +1435,7 @@ some point in the future.
       The *data* argument has the same meaning as the *data* argument of
       :func:`urlopen`.
 
+      This method always quotes *fullurl* using :func:`~urllib.parse.quote`.
 
    .. method:: open_unknown(fullurl, data=None)