bpo-29651 - Cover edge case of square brackets in urllib docs (#1128)

Doc/library/urllib.parse.rst

@@ -121,6 +121,9 @@ or on combining URL components into a URL string.
    an invalid port is specified in the URL.  See section
    :ref:`urlparse-result-object` for more information on the result object.
 
+   Unmatched square brackets in the :attr:`netloc` attribute will raise a
+   :exc:`ValueError`.
+
    .. versionchanged:: 3.2
       Added IPv6 URL parsing capabilities.
 
@@ -239,6 +242,9 @@ or on combining URL components into a URL string.
    an invalid port is specified in the URL.  See section
    :ref:`urlparse-result-object` for more information on the result object.
 
+   Unmatched square brackets in the :attr:`netloc` attribute will raise a
+   :exc:`ValueError`.
+
    .. versionchanged:: 3.6
       Out-of-range port numbers now raise :exc:`ValueError`, instead of
       returning :const:`None`.