Add comments for OrderedInstruction. NFC

llvm-svn: 306201

llvm/include/llvm/Transforms/Utils/OrderedInstructions.h

@@ -43,6 +43,9 @@ public:
   bool dominates(const Instruction *, const Instruction *) const;
 
   /// Invalidate the OrderedBasicBlock cache when its basic block changes.
+  /// i.e. If an instruction is deleted or added to the basic block, the user
+  /// should call this function to invalidate the OrderedBasicBlock cache for
+  /// this basic block.
   void invalidateBlock(BasicBlock *BB) { OBBMap.erase(BB); }
 };