further comment clarification

src/runtime/cgocall.go

@@ -530,8 +530,9 @@ const cgoResultFail = "cgo result is unpinned Go pointer or points to unpinned G
 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
 // is either a pointer to the value (of type t), or the value itself,
 // depending on indir. The top parameter is whether we are at the top
-// level, where Go pointers are allowed. Go pointers to pinned objects are
-// always allowed.
+// level, where Go pointers are allowed. Go pointers to pinned objects that
+// don't reference other unpinned pointers (enforced by cgoCheckUnknownPointer)
+// are always allowed.
 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
 	if t.PtrBytes == 0 || p == nil {
 		// If the type has no pointers there is nothing to do.