Document how to delete all tickets in Open Match

[sawagh]

We have seen a common pattern (typically when a Frontend crashes) where there is a desire to cleanup Open Match state. While Open Match does not provide a 'delete all' option, It does have recommendations based on some APIs (QueryTicketIDs) on how to delete all Tickets.

It will be useful to have the Open Match documentation state how this can be done with existing APIs to simplify it for the users.

See: https://github.com/googleforgames/open-match/issues/764