Document the "field present" and "field absent" operators in Ferret
Document the "field present" and "field absent" operators in Ferret

Summary: Ref T13509. Adds documentation for the new operators.

Test Plan: Read documentation, tried examples, got sensible-seeming results.

Maniphest Tasks: T13509

Differential Revision: https://secure.phabricator.com/D21112

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