[RISCV][doc] Improve documentation comments on atomics intrinsics

Previously, it was necessary to check the atomics lowering or expansion
code to determine which argument was which.

This patch additionally tweaks the documentation comment in
TargetLowering to clarify the return value of the intrinsic and that the
intrinsic isn't required to mask and shift the result (this is handled
by the target-independent code in AtomicExpandPass).