Docs - expand documentation on custom fields for selects a bit

Summary: ...and fix an error where lines that start with ##X## are rendering incorrectly by switching to alternate syntax `X`. Fixes T5806.

Test Plan: read the docs and they looked good

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5806

Differential Revision: https://secure.phabricator.com/D10165