Added a documentation category for statement attributes so that things like clang::fallthrough can be documented.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201714 91177308-0d34-0410-b5e6-96231b3b80d8
Moving the documentation for the clang::fallthrough attribute into AttrDocs.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201715 91177308-0d34-0410-b5e6-96231b3b80d8
Moving the documentation for the ARM interrupt attribute into AttrDocs.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201716 91177308-0d34-0410-b5e6-96231b3b80d8