Moving the documentation for the enable_if attribute into AttrDocs. Removed the "clang introduces" phrase for style consistency, but otherwise the documentation is identical.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201677 91177308-0d34-0410-b5e6-96231b3b80d8
Moving the documentation for the overloadable attribute into AttrDocs.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201678 91177308-0d34-0410-b5e6-96231b3b80d8