Fix two minor display issues with the Conduit "*.search" API documentation
Depends on D20685. Ref T13350. Currently:
- When a SearchEngine parameter is marked as hidden from Conduit, we may still render a table of possible values. Instead, only render the table if the parameter is actually usable.
- The table header is hard-coded to say 'statuses', which is just a silly mistake. (Most commonly, this table does have statuses constants.)
Test Plan: Viewed the Conduit API documentation for the new "slowvote.poll.search" API method, saw more sensible display behavior.
Maniphest Tasks: T13350
Differential Revision: https://secure.phabricator.com/D20686