Added a little more description to 'forever'

The forever section is unclear at present because there are two callback functions in play (the second argument in forever, and the first argument for fn), and also because it's not made clear that execution will stop if next is not called or if next is called with an argument.

This change is my suggestion to make it a little clearer.
Merge remote-tracking branch 'kybernetikos/patch-1'

Conflicts:
	README.md