Write a search user guide

Summary: This covers some of the basics and explains the not-entirely-obvious rules for global search.

Test Plan:

• Read document.
• Clicked new help menu link.
• Clicked link from "advanced function help".

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12517