Add method description to conduit.query

Summary: As suggested in T6950, add the method description to the response from conduit.query.

Test Plan: Called echo '{}' | arc call-conduit conduit.query and verified that the response contained the method description.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11467