cbepctl: Peformance Tuning Options (thread) can only be tuned down

See http://developer.couchbase.com/documentation/server/4.0/cli/cbepctl/set-flush_param.html

and

http://docs.couchbase.com/admin/admin/CLI/cbepctl-intro.html

The max_num_readers, max_num_writers, max_num_auxio and max_num_nonio values can only be tuned down, ie if at initial start-up they had value N then only a value < N will work.

The example is max_num_writers sets the number of writer threads to five (5).
We default at start-up to 4, so although it updates the parameter - it actually has no affect. In the example would be better choosing a value < 4.
Also need to highlight that can only tune these values down.

A separate MB will be raised to ensure in the tool only accepts values only lower than the initial defaults at start-up.

This relates to http://issues.couchbase.com/browse/MB-14761