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