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