Summary: There was some documentation for this but it was kind of buried in a
random, difficult-to-discover file. Separate it into its own file and link to it
from the previous location.
Test Plan: Regenerated documentation and read through it without catching
anything terrible.
Reviewers: btrahan
Reviewed By: btrahan
CC: zeeg, aran, btrahan
Maniphest Tasks: T643
Differential Revision: 1161
