Only link local titles on Special:Search

Anything else is confusing, especially given the message text
we have there.

Example:
https://www.mediawiki.org/wiki/Special:Search?search=en%3Afoo&fulltext=1

Also fix the documentation of Title::isLocal as that was misleading
and fooled me (in PS1).

Change-Id: Iccb69de4c7243bf144a12deb8b86505e56a111dd
Merge "Only link local titles on Special:Search"