Add a note to NEWS about Parrot_PMC_destroy

This information should go somewhere, probably in api.yaml as well. If
you remove this info from NEWS, please make sure it is recorded
somewhere else.

NEWS

@@ -1,5 +1,7 @@
 New in 3.5.0
 - Core
+  + Parrot_PMC_destroy was removed from the public extension API, due to it
+    not working correctly and it being part of GC internals.
 - Languages
 - Tests
   + The Parrot test suite harness now understands the HARNESS_TIMER environment