Fix outdated comment; all running bgworkers are in BackendList
Before commit
8a02b3d732, only bgworkers that connected to a database
had an entry in the Backendlist. Commit
8a02b3d732 changed that, but
forgot to update this comment.
Discussion: https://www.postgresql.org/message-id/
835232c0-a5f7-4f20-b95b-
5b56ba57d741@iki.fi