Moving the documentation for the type safety checking attributes into AttrDocs. If a custom heading is provided, do not automatically generate the alternate spelling list. This is necessary because some attributes have distinct semantic spellings and meanings, but use the same semantic attribute internally. Such attributes should have multiple elements in their documentation list, but not show all spellings. At some point, it would be nice to have a way to attach the documentation element to a specific spelling for these cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201851 91177308-0d34-0410-b5e6-96231b3b80d8