docs: Add learnings about _services.copy() fix to preview.py (#37956)

Capturing what we learned in https://github.com/openedx/openedx-platform/pull/37900#issuecomment-3811194293

cms/djangoapps/contentstore/views/preview.py

@@ -200,6 +200,17 @@ def _prepare_runtime_for_preview(request, block):
         # See the docstring of `DjangoXBlockUserService`.
         deprecated_anonymous_user_id = anonymous_id_for_user(request.user, None)
 
+    # NOTE: As of Ulmo, these services only apply to the preview views. If you want a service to be present in all
+    # Studio ModuleStoreRuntimes, then add it to load_services_for_studio.
+    # HISTORICAL CONTEXT: Until Ulmo, the `block.runtime._services.update(service)` call below would
+    # actually update the services dictionary for all runtimes, as `_services` was aliased between them.
+    # This caused a grading bug, under certain conditions, so it was fixed
+    # in https://github.com/openedx/openedx-platform/pull/37825; now, every runtime gets a fresh,
+    # independent copy of `_services`. That's good, except that some Studio code had become dependent
+    # on the bugged behavior and thus expected the "preview" services below to be present in all Studio runtimes.
+    # We fixed the known instance of that bugged assumption here:
+    # https://github.com/openedx/openedx-platform/pull/37900.
+    # This comment is left here as a note for future devs investigating similar bugs.
     services = {
         "studio_user_permissions": StudioPermissionsService(request.user),
         "i18n": XBlockI18nService,