Clean up "phabricator.timezone" configuration instructions a little bit

Summary: These instructions are fairly old and can be a little fancier and more clear in the context of modern Phabricator. Drop the reference to "HPHP", link the actual timezone list, wordsmith a little.

Test Plan: d( O_o )b

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20560