Document upgrade and compatibility policy

- the order of upgrades of backend components is always: query, collector, agents
- compatibility guarantee between backend components: V-1 and V only
- compatibility between clients and backends: by default backend should be compatible with all clients, but there may be future exceptions

Start by locating the Jaeger documentation page covering backend upgrades and compatibility; no specific file or test is named in the issue. Document the upgrade order, the V-1/V backend guarantee, and the default client-backend compatibility policy, including the noted possibility of future exceptions.

Written by the indexing model from the issue text.