Add clarifying comments defining what a Loop is

Our loop construct is not a way to identify cycles in the CFG.  This wasn't immediately obvious from the header, so clarify that fact.

The motivation for this was that I just fixed a out of tree bug due to a mistaken assumption (on my part) on what a Loop actually was.  While it was fresh in my mind, I wanted to document the key point.

llvm-svn: 258154