[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

clang/include/clang/AST/DeclObjC.h

@@ -137,6 +137,9 @@ public:
 /// the above methods are setMenu:, menu, replaceSubview:with:, and defaultMenu.
 ///
 class ObjCMethodDecl : public NamedDecl, public DeclContext {
+  // This class stores some data in DeclContext::ObjCMethodDeclBits
+  // to save some space. Use the provided accessors to access it.
+
 public:
   enum ImplementationControl { None, Required, Optional };
 
@@ -953,6 +956,9 @@ public:
 /// ObjCProtocolDecl, and ObjCImplDecl.
 ///
 class ObjCContainerDecl : public NamedDecl, public DeclContext {
+  // This class stores some data in DeclContext::ObjCContainerDeclBits
+  // to save some space. Use the provided accessors to access it.
+
   // These two locations in the range mark the end of the method container.
   // The first points to the '@' token, and the second to the 'end' token.
   SourceRange AtEnd;