transcribe.mdx: clarify that no text is returned on calling.transcript.failed

Author: hey-august

fern/products/swml/pages/reference/methods/calling/transcribe.mdx

@@ -41,7 +41,7 @@ Only one transcription can be active on a call at a time. Starting another while
 
 ## **StatusCallbacks**
 
-When you set `status_url`, SignalWire POSTs a transcript event to it when the call's transcription finishes: `calling.transcript.completed` with the transcribed `text` on success, or `calling.transcript.failed` if the call could not be transcribed.
+When you set `status_url`, SignalWire POSTs a transcript event to it when the call's transcription finishes: `calling.transcript.completed` includes the transcribed `text` when speech was captured, or `calling.transcript.failed` if the call could not be transcribed.
 
 <WebhookPayloadSnippet webhook="transcribeStatusCallback" />