Asynchronous Uploads (#15503)

Support asynchronous uploads as defined in MSC2246.

docs/usage/configuration/config_documentation.md

@@ -1753,6 +1753,19 @@ rc_third_party_invite:
   burst_count: 10
 ```
 ---
+### `rc_media_create`
+
+This option ratelimits creation of MXC URIs via the `/_matrix/media/v1/create`
+endpoint based on the account that's creating the media. Defaults to
+`per_second: 10`, `burst_count: 50`.
+
+Example configuration:
+```yaml
+rc_media_create:
+  per_second: 10
+  burst_count: 50
+```
+---
 ### `rc_federation`
 
 Defines limits on federation requests.
@@ -1814,6 +1827,27 @@ Example configuration:
 media_store_path: "DATADIR/media_store"
 ```
 ---
+### `max_pending_media_uploads`
+
+How many *pending media uploads* can a given user have? A pending media upload
+is a created MXC URI that (a) is not expired (the `unused_expires_at` timestamp
+has not passed) and (b) the media has not yet been uploaded for. Defaults to 5.
+
+Example configuration:
+```yaml
+max_pending_media_uploads: 5
+```
+---
+### `unused_expiration_time`
+
+How long to wait in milliseconds before expiring created media IDs. Defaults to
+"24h"
+
+Example configuration:
+```yaml
+unused_expiration_time: "1h"
+```
+---
 ### `media_storage_providers`
 
 Media storage providers allow media to be stored in different