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>

doc/api/cluster.md

@@ -587,9 +587,6 @@ values are `"rr"` and `"none"`.
 After calling `.setupMaster()` (or `.fork()`) this settings object will contain
 the settings, including the default values.
 
-It is effectively frozen after being set, because `.setupMaster()` can
-only be called once.
-
 This object is not supposed to be changed or set manually, by you.
 
 ## cluster.setupMaster([settings])