Document configuration of file upload limits

Summary: I have a patch which makes uploads all fancy and adds progress bars, but document the landscape first since it's quite complicated.

Test Plan: Generated, read docs. Configured storage.upload-size-limit to various values.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T875

Differential Revision: https://secure.phabricator.com/D2381