Change title of section to reflect the concept, and not the mechanism.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193277 91177308-0d34-0410-b5e6-96231b3b80d8
Provide documentation on attribute((objc_requires_super)).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193278 91177308-0d34-0410-b5e6-96231b3b80d8