Document media hashing changes (#18296)

Essentially document the change in behaviour in #18277 

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changelog.d/18296.doc

@@ -0,0 +1 @@
+Hashes of media files are now tracked by Synapse. Media quarantines will now apply to all files with the same hash.

docs/admin_api/media_admin_api.md

@@ -46,6 +46,14 @@ to any local media, and any locally-cached copies of remote media.
 
 The media file itself (and any thumbnails) is not deleted from the server.
 
+Since Synapse 1.128.0, hashes of uploaded media are tracked. If this media
+is quarantined, Synapse will:
+
+ - Quarantine any media with a matching hash that has already been uploaded.
+ - Quarantine any future media.
+ - Quarantine any existing cached remote media.
+ - Quarantine any future remote media.
+
 ## Quarantining media by ID
 
 This API quarantines a single piece of local or remote media.