R1:4d2210a9f09a

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