Docs for SERVER-6558: findAndModify should take a write concern

Engineering Ticket Description:

The findAndModify should support specification of a writeConcern, in the same manner as the write commands (insert, update, delete), and should support returning write concern failures in the same manner, as well.

EDIT 8/31/15
This ticket originally encompassed work to support sending write concern for all commands, but the work committed for this ticket is only for the findAndModify command.

The ticket describing the work for extending support for this to all commands which write can be found in SERVER-20224.

All commands that perform writes on behalf of users should support specification of a writeConcern, in the same manner as the write commands (insert, update, delete), and should support returning write concern failures in the same manner, as well.