Remove Phabricator reference from the linter description of `ArcanistXHPASTLinter`

Summary: The `ArcanistXHPASTLinter` is no longer //too// specific to Phabricator (Phabricator-specific XHPAST lints generally live in `ArcanistPhutilXHPASTLinter`) and //should// be better suited for general purpose usage.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9892