doc: warn about SuppressDestruct()

Not sure how this ever made its way into the API, but people should *never* use this.

Author: addaleax

napi.h

@@ -1167,6 +1167,8 @@ namespace Napi {
     // Call this on a reference that is declared as static data, to prevent its destructor
     // from running at program shutdown time, which would attempt to reset the reference when
     // the environment is no longer valid.
+    // Avoid using this if at all possible. If you do need to use static data, 
+    // MAKE SURE to warn your users that your addon is NOT threadsafe.
     void SuppressDestruct();
 
   protected: