nixos/prosody: provide additional details in the user and group options description

nixos/modules/services/networking/prosody.nix

@@ -529,13 +529,29 @@ in
       user = mkOption {
         type = types.str;
         default = "prosody";
-        description = "User account under which prosody runs.";
+        description = ''
+          User account under which prosody runs.
+
+          <note><para>
+          If left as the default value this user will automatically be created
+          on system activation, otherwise you are responsible for
+          ensuring the user exists before the prosody service starts.
+          </para></note>
+        '';
       };
 
       group = mkOption {
         type = types.str;
         default = "prosody";
-        description = "Group account under which prosody runs.";
+        description = ''
+          Group account under which prosody runs.
+
+          <note><para>
+          If left as the default value this group will automatically be created
+          on system activation, otherwise you are responsible for
+          ensuring the group exists before the prosody service starts.
+          </para></note>
+        '';
       };
 
       allowRegistration = mkOption {