feat(docs): Add a note that token context doesn't accept number or null

doc/tokens.md

   - 'name' is the display name of the user
   - 'email' is the email of the user
   - 'avatar' is the URL of the avatar for the user
+> Note: As the moment all fields in 'user' need to be a valid string, numeric types or `null` will generate an exception.
 - 'callee' is an optional object containing display information when launching a 1-1 video call with a single other participant.  It used to display an overlay to the first user, before the second user joins.
   - 'id' is a user identifier string.  Intended for use in reporting/analytics
   - 'name' is the display name of the 'callee' user