Add a couple of clarifying comments to the Mercurial protocol parser

Summary: See D18857. Ref T13036. See PHI275. Explain what's going on here a little better since it isn't entirely obvious and debugging these stream parsers is a gigantic pain.

Test Plan: Read text.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13036

Differential Revision: https://secure.phabricator.com/D18859