Improve some documentation for "diffusion.commit.search"

Summary:
Ref T13195. See PHI851. Start by making some minor improvements here:

• Clarify that the example of what constraints look like is just an example.
• Add descriptions for parameters missing descriptions.

Test Plan: Looked at API method page, saw more helpful/complete instructions.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19640