Document 3-arg overloads of Q*Atomic*::testAndSet(Relaxed|Acquire|Release|Ordered)

Ever since the 3-arg testAndSet overloads were added, they should have been the go-to version for most users, because they save a load, as they return the value found in the atomic variable for free.

But they're not documented, so users (and Qt developers alike) don't know they exist.

Document these four functions on Q(Basic)Atomic(Integer|Pointer).

I think they're also missing from fake class definition in qatomic.h, so they'd need to be added there, too.