doc: clarify cluster behaviour with no workers

Fixes: nodejs#1239
Ref: 41b75ca
PR-URL: nodejs#2606
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

2015-08-25 io.js v3.2.0 Release

2015-08-19 io.js v3.1.0 Release

2015-08-04 io.js v3.0.0 Release

2015-08-03 io.js v3.0.0 RC 7

2015-07-30 io.js v3.0.0 RC 6

2015-07-28 io.js v2.5.0 Release

2015-07-24 io.js v3.0.0 RC 5

2015-07-22 io.js v3.0.0 RC 4

doc: Add notes for using @@toStringTag