Docs: engine.execute reports a failed workspace save through workspaceStorageError (#696)

docs/control-protocol.md

@@ -82,9 +82,11 @@ preserved for the entire session, including after later layout changes. Load and
 appear in the status message and the nullable
 `workspaceStorageError` field; a failed write remains eligible for retry.
 `window.saveWorkspace`, `window.saveWorkspaceAs` and `window.editWorkspaces` (rename, delete)
-write the file immediately and return an error when it isn't written (a failed write, a file
-preserved after a failed load, or no configuration store). The changed workspaces still apply
-for the session, and a later save retries them.
+write the file immediately. Through `ui.menu.invoke` they return an error when it isn't written
+(a failed write, a file preserved after a failed load, or no configuration store);
+`engine.execute` hands them to the UI and returns `{frontend}` first, so check
+`workspaceStorageError` there. The changed workspaces still apply for the session, and a later
+save retries them.
 
 ## Input