Update migration guide with multi-tenancy changes

doc/migrations/4.2.0_5.0.0.md

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+## DB migrations
+
+The migrations scripts for Postgres, MySQL, MSSQL can be found in the [`priv/migrations`](../../priv/migrations/) directory.
+
+### Changes in XEPs:
+- `mod_last` 
+   - Table `last` - added server column, updated primary key and indexes.
+- `mod_privacy` 
+   - Table `privacy_default_list` - added server column, updated primary key and indexes.
+   - Table `privacy_list` - added server column, updated primary key and indexes.
+- `mod_private`
+   - Table `private_storage` - added server column, updated primary key and indexes, removed unused columns.
+- `mod_roster` 
+   - Table `rosterusers` - added server column, updated indexes.
+   - Table `rostergroups` - added server column, updated indexes.
+   - Table `roster_version` - added server column, updated primary key and indexes.
+- `mod_muc` 
+   - Table `i_muc_light_blocking` - updated indexes.
+- `mod_inbox` 
+   - Table `inbox` - updated primary key and indexes.
+
+### Other changes:
+- RDBMS auth - modified `users` table.
+- Added `domain_settings` table.
+- Added `domain_events` table.
+
+## Hook migrations
+
+- `filter_room_packet` hook uses a map instead of a proplist
+  for the event data information.
+- `room_send_packet` hook has been removed. Use `filter_room_packet` instead.
+- `filter_room_packet` is called for HostType (was for MucHost).
+- `forget_room` is called for HostType (was for MucHost).
+- `forget_room` takes an extra argument HostType.
+- `filter_room_packet` takes an extra argument HostType.
+- `is_muc_room_owner` is called for HostType (was for MucHost).
+- `is_muc_room_owner` takes an extra argument HostType.
+- `muc_room_pid` hook removed.
+- `load_permanent_rooms_at_startup` option is ignored now.
+- `gen_mod:get_module_opt_by_subhost` API removed.
+- `update_inbox_for_muc` is called for HostType.
+- `get_mam_muc_gdpr_data` is called for HostType.
+- `get_mam_pm_gdpr_data` is called for HostType.
+- `get_personal_data` handlers take an extra argument: `HostType` as the second parameter.
+- `get_mam_pm_gdpr_data` and `get_mam_muc_gdpr_data` take `HostType` argument.
+
+## Metrics REST API (obsolete)
+
+The API is still considered obsolete so if you are using it,
+please consider using [WombatOAM](https://www.erlang-solutions.com/capabilities/wombatoam/)
+or metrics reporters as described in [Logging and monitoring](../operation-and-maintenance/Logging-&-monitoring.md).
+
+In each endpoint, `host` has been changed to `host_type`.
+This is because the metrics are now collected per host type rather than host.
+
+
+## Users cache
+
+MongooseIM used to feature a cache to check whether a user exists, that was unavoidably enabled, and had no eviction policy, that is, the cache could continue growing forever. Now, MIM features a module called [`mod_cache_users`](../modules/mod_cache_users) that implements a configurable cache policy, that can be enabled, disabled, and parametrised, per `host_type`. This might not be enabled by default in your configuration, so we recommend you verify your configuration and enable it if needed.