Fix two minor display issues with the Conduit "*.search" API documentation

Summary:
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