AttributeReference.rst is automatically generated by a server-side process currently. To cut down on accidental commits to this file that are not properly reflected in AttrDocs.td (such as r215806 - r215808), this file now contains nothing but a comment explaining the current state of affairs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240428 91177308-0d34-0410-b5e6-96231b3b80d8