Comment parsing: add more comments to CommentCommands.td

I hope the ASCII art delimiters are OK, since they group *groups* of commands --
that is really helpful.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174114 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed