Document per-user and ephemeral vars

Substitutions.md

@@ -9,6 +9,15 @@ notations can be used to include this information in your messages:
 
 * $varname$ - variables created by and for your own channel. For example,
   you might have a counter for the number of times you've died: $deaths$
+* $*varname$ - variables associated with the current user (within this
+  channel). These are tracked separately for all users that use them.
+* $kwd*varname$ - variables associated with a specific user - see the
+  "User Vars" builtin to specify the user
+* $varname?$ - ephemeral variables which can be accessed identically to
+  others, but are not saved across bot restarts. These may disappear at
+  any time, but will usually stick around for months. They will not be
+  shown in the /variables page for the channel. Note that this can be
+  combined with per-user notation also, creating a per-user ephemeral.
 * {token} - information provided by a builtin or special trigger. When a
   command calls on a builtin, it will receive a specific set of keywords
   which can be used in this way.