R1:e9b861ff1583

Write a basic Calendar user guide

Summary:
Ref T11809. Roughly documents most of the tricky/unintuitive stuff.

Also fixes a bug with "Make Recurring" with no "Until" date.

Test Plan: Read document.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16792