Documentation: update 'cluster'

[gblazex]

Stability: 2 - Stable

vs.

This feature was introduced recently, and may change in future versions. Please try it out and provide feedback.

https://nodejs.org/api/cluster.html

[Fishrock123]

@galambalazs We'd definitely take a Pull Request about it! :)

[Fishrock123]

Fixed in b9279aa :)