[AST][NFC] Add missing doc for ObjCMethodDecl and ObjCContainerDecl

Add a comment in ObjCMethodDecl and ObjCContainerDecl stating that
we store some bits in ObjCMethodDeclBits and ObjCContainerDeclBits.

This was missed by the recent move in
r338641 : [AST][4/4] Move the bit-fields from ObjCMethodDecl
          and ObCContainerDecl into DeclContext

llvm-svn: 338888