[embedded] Add a more detailed explaining comment to swift_release_n_

Author: kubamracek

stdlib/public/core/EmbeddedRuntime.swift

@@ -292,7 +292,14 @@ func swift_release_n_(object: UnsafeMutablePointer<HeapObject>?, n: UInt32) {
   let resultingRefcount = subFetchAcquireRelease(refcount, n: Int(n)) & HeapObject.refcountMask
   if resultingRefcount == 0 {
     // Set the refcount to immortalRefCount before calling the object destroyer
-    // to prevent future retains/releases from having any effect.
+    // to prevent future retains/releases from having any effect. Unlike the
+    // full Swift runtime, we don't track the refcount inside deinit, so we
+    // won't be able to detect escapes or over-releases of `self` in deinit. We
+    // might want to reconsider that in the future.
+    //
+    // There can only be one thread with a reference at this point (because
+    // we're releasing the last existing reference), so a relaxed store is
+    // enough.
     storeRelaxed(refcount, newValue: HeapObject.immortalRefCount)
 
     _swift_embedded_invoke_heap_object_destroy(object)