Minor modification to `PhutilJSONParser` documentation

Summary: The `PhutilJSONParser` class is reasonably stable, we no longer need to advise against using it.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9853