Fix/clarify documentation in GHC.Stats
The following were not clear to me until I read the source:
numByteUsageSamples is (also) the number of major GCs performed.
currentBytesUsed is the number of live bytes at the end of the last major GC.
currentBytesSlop also at the end of the last major GC? Can't see that it's actually set to anything nonzero anywhere...
The comments for
parTotBytesCopied are in the wrong places.