Untrusted devices may delete data

Explicitly mention that untrusted devices can delete files from a
synchronised folder. While this can be inferred from the current
description, it's probably best to be explicitly for this kind of
caveat.

Author: WhyNotHugo

users/untrusted.rst

@@ -83,6 +83,9 @@ Similarly, it's fine to add "normal mode" synchronization between untrusted devi
         }
     }
 
+In both of the above scenarios, an untrusted device may delete files being
+synchronised, and the deletion shall be replicated to both `T1` and `T2`.
+
 Configuration
 -------------
 
@@ -144,6 +147,10 @@ and receive the data for the new file over the network. However if you have a bi
 e.g. ``video.mp4``, and you modify just a part of it (e.g. video metadata), only
 the changed block is transferred as usual.
 
+Untrusted devices may delete files locally. When this synchronises onto other
+hosts, the corresponding plaintext counterpart of that file is also deleted and
+lost.
+
 .. _untrusted-decrypt:
 
 Decrypting data