Fixed presence results not returning offline users on initial sync (#17231)

This is to address an issue in which `m.presence` results on initial
sync are not returning entries of users who are currently offline.

The original behaviour was from
https://github.com/element-hq/synapse/issues/1535

This change is useful for applications that use the
presence system for tracking user profile information/updates (e.g.
https://github.com/element-hq/synapse/pull/16992 or for profile status
messages).

This is gated behind a new configuration option to avoid performance
impact for applications that don't need this, as a pragmatic solution
for now.

docs/usage/configuration/config_documentation.md

@@ -246,6 +246,7 @@ Example configuration:
 ```yaml
 presence:
   enabled: false
+  include_offline_users_on_sync: false
 ```
 
 `enabled` can also be set to a special value of "untracked" which ignores updates
@@ -254,6 +255,10 @@ received via clients and federation, while still accepting updates from the
 
 *The "untracked" option was added in Synapse 1.96.0.*
 
+When clients perform an initial or `full_state` sync, presence results for offline users are
+not included by default. Setting `include_offline_users_on_sync` to `true` will always include
+offline users in the results. Defaults to false.
+
 ---
 ### `require_auth_for_profile_requests`