Improve default_power_level_content_override documentation (#17451)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/17451.doc

@@ -0,0 +1 @@
+Improve documentation for the [`default_power_level_content_override`](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#default_power_level_content_override) config option.

docs/usage/configuration/config_documentation.md

@@ -4134,6 +4134,38 @@ default_power_level_content_override:
    trusted_private_chat: null
    public_chat: null
 ```
+
+The default power levels for each preset are:
+```yaml
+"m.room.name": 50
+"m.room.power_levels": 100
+"m.room.history_visibility": 100
+"m.room.canonical_alias": 50
+"m.room.avatar": 50
+"m.room.tombstone": 100
+"m.room.server_acl": 100
+"m.room.encryption": 100
+```
+
+So a complete example where the default power-levels for a preset are maintained
+but the power level for a new key is set is:
+```yaml
+default_power_level_content_override:
+   private_chat:
+    events:
+      "com.example.foo": 0
+      "m.room.name": 50
+      "m.room.power_levels": 100
+      "m.room.history_visibility": 100
+      "m.room.canonical_alias": 50
+      "m.room.avatar": 50
+      "m.room.tombstone": 100
+      "m.room.server_acl": 100
+      "m.room.encryption": 100
+   trusted_private_chat: null
+   public_chat: null
+```
+
 ---
 ### `forget_rooms_on_leave`
 

synapse/handlers/room.py

@@ -1188,6 +1188,8 @@ async def create_event(
             )
             events_to_send.append((power_event, power_context))
         else:
+            # Please update the docs for `default_power_level_content_override` when
+            # updating the `events` dict below
             power_level_content: JsonDict = {
                 "users": {creator_id: 100},
                 "users_default": 0,