doc: remove cluster.setupMaster() myth

cluster.setupMaster() can be called more than once. Core even has
tests for this functionality. This commit removes an incorrect
statement to the contrary from the documentation.

Fixes: https://github.com/nodejs/node/issues/7156
PR-URL: https://github.com/nodejs/node/pull/7179


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>