docs: Add missing docs for function(keep)

[VorpalBlade]

Fixes #754

I have not made any fix for 0.13. Don't know if we want to backport it (should be very simple if so)

[VorpalBlade]

Merged your changes (on the phone at the moment, and since you apparently squash merge anyway it doesn't matter that I normally try to make clean mergable histories)

[udoprog]

Thanks!