Add comments to XHPAST functions
Add comments to XHPAST functions

Summary: Add some docblock comments to various XHPAST functions.

Test Plan: Eyeball it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11519

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil