R1:77d7bb7af0a2

Document the Ferret "=" operator and improve related documentation

Summary:
Depends on D19529. See PHI778.

- Document the "name" constraint as deprecated. All callers are likely better served by the "query" constraint.
- Guide users toward the "query" constraint a little better.
- Document the `=` syntax.

Test Plan: Read various new documentation.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19531