chore: note Small add-on requirement for PITR (supabase#18689)

Author: darora

apps/docs/pages/guides/platform/backups.mdx

@@ -72,6 +72,8 @@ Point-in-Time Recovery (PITR) allows a project to be backed up at much shorter i
 
 This feature is available to all Enterprise plan projects. Pro plan projects can enable PITR as an add-on.
 
+Projects interested in PITR will also need to use at least a Small compute add-on, in order to ensure smooth functioning.
+
 </Admonition>
 
 {mutuallyExclusiveNote}