Until attribute documentation is implemented as a server-side feature, I will be manually committing AttributeReference.rst so that attribute documentation can be on the live server instead of simply disappearing from the language extensions page without obvious replacement.

This is a temporary stop-gap solution until server-side generation is implemented, at which point the AttributeReference.rst will go back to holding placeholder text.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201733 91177308-0d34-0410-b5e6-96231b3b80d8