Bug 1534440 - Document GCPolicy<T>::isValid r=jonco

Differential Revision: https://phabricator.services.mozilla.com/D23046

--HG--
extra : moz-landing-system : lando

js/public/GCPolicyAPI.h

@@ -25,6 +25,11 @@
 //         from the container. Specializing this method is the standard way to
 //         get custom weak behavior from a container type.
 //
+//   static bool isValid(const T& t)
+//       - Return false only if |t| is corrupt in some way. The built-in GC
+//         types do some memory layout checks. For debugging only; it is ok
+//         to always return true.
+//
 // The default GCPolicy<T> assumes that T has a default constructor and |trace|
 // and |needsSweep| methods, and forwards to them. GCPolicy has appropriate
 // specializations for pointers to GC things and pointer-like types like