Add comment about cache misses on trivial changes

src/kernel/mpipeline.ml

@@ -272,6 +272,9 @@ let process
           if config.merlin.use_ppx_cache then None
           else Some "configuration"
         in
+        (* With this, the cache is invalidated even for source changes that don't
+           change the parsetree. To avoid that, we'd have to digest the parsetree
+           in the cache. *)
         let force_invalidation = not source_is_unmodified in
         Ppx_with_cache.apply ~cache_disabling ~force_invalidation {parsetree; config}
       in