Tweak handling of "Welcome" and "[x](y)" where "y" has parentheses
- [[ / ]] is better as a wiki root link than an install root link.
- Let [x](y) work when y contains balanced, singly-nested parentheses.
Test Plan: Used both kinds of markup in documents, saw appropriate rendering
Differential Revision: https://secure.phabricator.com/D19111