Add note about using `CurrentThreadIdentityIfPresent()` on darwin based platforms.

PiperOrigin-RevId: 515353041
Change-Id: I2c88022d50a3351f70d09a2d63020470889752d9

absl/base/internal/thread_identity.h

@@ -170,7 +170,10 @@ struct ThreadIdentity {
 //
 // Does not malloc(*), and is async-signal safe.
 // [*] Technically pthread_setspecific() does malloc on first use; however this
-// is handled internally within tcmalloc's initialization already.
+// is handled internally within tcmalloc's initialization already. Note that
+// darwin does *not* use tcmalloc, so this can catch you if using MallocHooks
+// on Apple platforms. Whatever function is calling your MallocHooks will need
+// to watch for recursion on Apple platforms.
 //
 // New ThreadIdentity objects can be constructed and associated with a thread
 // by calling GetOrCreateCurrentThreadIdentity() in per-thread-sem.h.