Document media hashing changes (#18296)

Essentially document the change in behaviour in #18277 

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1 files changed, 8 insertions, 0 deletions

diff --git a/docs/admin_api/media_admin_api.md b/docs/admin_api/media_admin_api.md

index 30833f3109..1177711c1e 100644
--- a/docs/admin_api/media_admin_api.md
+++ b/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.