Added comment to assertion regarding to bref->bref_pending_cmd which is a one-slot buffer for database query. If sessoin command cursor is active when query is routed, instead of routing the query to backend it is stored to bref_pending_cmd to wait until previous (session) command is completed. The assertion traps if there is a command already in the bref_pending_cmd.

Situation is wrong because client shouldn't send new query before the previous (the pending one) is executed in backend and the reply is sent to the client.
Merge branch 'release-1.0GA' of https://github.com/mariadb-corporation/MaxScale into release-1.0GA