Revise CK_NullToPointer comment

This addresses a post-commit review comment in #137364 (comment).

Author: alexfh

clang/include/clang/AST/OperationKinds.def

@@ -119,8 +119,7 @@ CAST_OPERATION(ArrayToPointerDecay)
 CAST_OPERATION(FunctionToPointerDecay)
 
 /// CK_NullToPointer - Null pointer constant to pointer, ObjC
-/// pointer, or block pointer. The result of this conversion can
-/// still be a null pointer constant if it has type std::nullptr_t.
+/// pointer, block pointer, or std::nullptr_t.
 ///   (void*) 0
 ///   void (^block)() = 0;
 CAST_OPERATION(NullToPointer)