Document that phd.taskmasters is a local setting, per daemon

Summary: Ref T13187. See PHI807. The documentation currently does not make it very clear that this is a local setting, per phd process. Make it more clear.

Test Plan: {F5827757}

Reviewers: amckinley

Maniphest Tasks: T13187

Differential Revision: https://secure.phabricator.com/D19597