Add user documentation for managing large changesets

Summary: See task and P91, this just adapts my Discussion post to be more
general. I'll follow up by linking to it from the Arcanist error message.
Test Plan: Read the document.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 701