[libTooling][NFC] Add a comment about comment parsing to getAssociatedRange.

It took me multiple hours of debugging plus asking an expert for help to
figure out why this function didn't do what it promised to do. It turns
out there is a flag that needs to be set. Document this, in an attempt
to save the next person the surprise.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D128774