fix(docs): Add notes for sms (#184), r=@shane-tomlinson

docs/api.md

@@ -102,13 +102,18 @@ Failing requests may be due to the following errors:
 ## POST /check
 
 Called by the auth server before performing an action on its end to
-check whether or not the action should be blocked.
+check whether or not the action should be blocked. The endpoint is
+capable of rate-limiting and blocking requests that involve a variety
+of [actions](https://github.com/mozilla/fxa-customs-server/blob/master/lib/actions.js).
 
 ___Parameters___
 
 * email - the email address associated with the account
 * ip - the IP address where the request originates
 * action - the name of the action under consideration
+* headers - the forwarded headers of the original request
+* payload - the payload of the original request
+ * phoneNumber - optional phone number of request
 
 ### Request