Made considerable updates to the documentation explaining how to add a new...

Made considerable updates to the documentation explaining how to add a new attribute to clang. Cleans up some of the existing wording, as well as adding new information and better explanations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237268 91177308-0d34-0410-b5e6-96231b3b80d8