docs(pgmq): clarify wording around reading messages (supabase#31699)

The original documentation made it sound like the `vt` parameter specified how long until the message became invisible, when in reality it is how long the message is invisible. See: https://github.com/tembo-io/pgmq/tree/main?tab=readme-ov-file#read-messages
> Read 2 message from the queue. Make them invisible for 30 seconds. If the messages are not deleted or archived within 30 seconds, they will become visible again and can be read by another consumer.

Author: bstncartwright

apps/docs/content/guides/queues/pgmq.mdx

@@ -207,7 +207,7 @@ select * from pgmq.send_batch(
 
 #### `read`
 
-Read 1 or more messages from a queue. The VT specifies the delay in seconds between reading and the message becoming invisible to other consumers.
+Read 1 or more messages from a queue. The VT specifies the duration of time in seconds that the message is invisible to other consumers. At the end of that duration, the message is visible again and could be read by other consumers.
 
 {/* prettier-ignore */}
 ```sql