DI: Make comments "brief"-er, NFC

Follow-up to r219801.  Post-commit review pointed out that all comments
require a `\brief` description [1], so I converted many and recrafted a
few to be briefer or to include a brief intro.  (If I'm going to clean
them up, I should do it right!)

[1]: http://llvm.org/docs/CodingStandards.html#doxygen-use-in-documentation-comments

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219808 91177308-0d34-0410-b5e6-96231b3b80d8