AttributeReference: Document __single_inhertiance, __multiple_inheritance, __virtual_inheritance

Add documentation for these attributes, it includes:
- Motivation for their existence.
- Examples on how to use them.
- Examples on how to misuse them.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@202121 91177308-0d34-0410-b5e6-96231b3b80d8