Introduce and document a new cluster.mailers option for configuring multiple mailers

Summary:
Depends on D19002. Ref T13053. Ref T12677. Adds a new option to allow configuration of multiple mailers.

Nothing actually uses this yet.

Test Plan: Tried to set it to various bad values, got reasonable error messages. Read documentation.

Reviewers: amckinley

Maniphest Tasks: T13053, T12677

Differential Revision: https://secure.phabricator.com/D19003