Merge pull request 'Mention that user perms may be empty sometimes.' (#2) from rust-moment into mistress

Reviewed-on: #2

Protocol.md

@@ -53,6 +53,8 @@ User permissions are a set of flags separated by either the form feed character
 The reason there are two options is due to a past mixup that we now have to live with.
 Which of the methods is used remains consistent per server however, so the result of a test can be cached.
 
+Note that this string MAY be empty if sent by the bot user (`-1`).
+
 | Type   | Description |
 |:------:| ----------- |
 | `int`  | Rank of the user. Used to determine what channels a user can access or what other users the user can moderate. |