Fix Documentation of Object.keyOf.

Object.keyOf returns `null` and not `false` when item not found.
merge: Documentation of Object.keyOf. (#2578)

Correction for Documentation of Object.keyOf

The documentation for Object.keyOf lists the return as false if a match
is not found. However, the function actually returns null in this
scenario. I assume that at this point changing the code would break
compatibility, and so instead suggest that the documentation be updated
to reflect the correct return value.

Thanks @jdmichal for reporting.

Closes #2578.