Clarify documentation of the admin list media API (#8795)

Clarify that the list media API only shows media from unencrypted events.

changelog.d/8795.doc

+Improve the documentation for the admin API to list all media in a room with respect to encrypted events.

docs/admin_api/media_admin_api.md

 # List all media in a room
 
 This API gets a list of known media in a room.
+However, it only shows media from unencrypted events or rooms.
 
 The API is:
 ```