Define some basic terminology around loops in our documentation

I've noticed a lot of confusion around this area recently with key terms being misused in a number of threads. To help reign that in, let's go ahead and document the current terminology and meaning thereof.

My hope is to grow this over time into a broader discussion of canonical loop forms - yes, there are more than one ... many more than one - but for the moment, simply having the key terminology is a good stopping place.

Note: I am landing this *without* an LGTM.  All feedback so far has been positive, and trying to apply all of the suggested changes/extensions would cause the review to never end.  Instead, I decided to land it with the obvious fixes made based on reviewer comments, then iterate from there.

Differential Revision: https://reviews.llvm.org/D65164

llvm-svn: 366960