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

(cherry picked from commit 2fb2bc81c3f40d73945c6102569495140e1182c7)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
diff --git a/Doc/library/urllib.request.rst b/Doc/library/urllib.request.rst
index 289bfca..1bc81e0 100644
--- a/Doc/library/urllib.request.rst
+++ b/Doc/library/urllib.request.rst
@@ -1435,6 +1435,7 @@
       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)