Define delayed event ratelimit category (#18019)

Apply ratelimiting on delayed event management separately from messages.

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---------

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

docs/usage/configuration/config_documentation.md

@@ -1947,6 +1947,29 @@ rc_presence:
     burst_count: 1
 ```
 ---
+### `rc_delayed_event_mgmt`
+
+Ratelimiting settings for delayed event management.
+
+This is a ratelimiting option that ratelimits
+attempts to restart, cancel, or view delayed events
+based on the sending client's account and device ID.
+It defaults to: `per_second: 1`, `burst_count: 5`.
+
+Attempts to create or send delayed events are ratelimited not by this setting, but by `rc_message`.
+
+Setting this to a high value allows clients to make delayed event management requests often
+(such as repeatedly restarting a delayed event with a short timeout,
+or restarting several different delayed events all at once)
+without the risk of being ratelimited.
+
+Example configuration:
+```yaml
+rc_delayed_event_mgmt:
+  per_second: 2
+  burst_count: 20
+```
+---
 ### `federation_rr_transactions_per_room_per_second`
 
 Sets outgoing federation transaction frequency for sending read-receipts,