Moving the documentation for the availability attribute into AttrDocs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201712 91177308-0d34-0410-b5e6-96231b3b80d8
Some of these headings had the incorrect number of "underlines" and so would get warnings when generating the content from Sphinx. No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201713 91177308-0d34-0410-b5e6-96231b3b80d8