PrototypeInfo.validity_cell: adding comment

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 075cca447b63d4ac3bdbb928d2458b0583906ed9..6f849584a8feb534877d4097ec87273d31244dc2 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5990,6 +5990,10 @@ class PrototypeInfo : public Struct {
   inline void set_registry_slot(int slot);
   // [validity_cell]: Cell containing the validity bit for prototype chains
   // going through this object, or Smi(0) if uninitialized.
+  // When a prototype object changes its map, then both its own validity cell
+  // and those of all "downstream" prototypes are invalidated; handlers for a
+  // given receiver embed the currently valid cell for that receiver's prototype
+  // during their compilation and check it on execution.
   DECL_ACCESSORS(validity_cell, Object)
   // [constructor_name]: User-friendly name of the original constructor.
   DECL_ACCESSORS(constructor_name, Object)