updated your guidelines for core node "inject"

Hi, i'm developing an "overlay" for node-red that enables a webapp to show dashboards for interacting with flows and edit a flow in a react webapp project.
The inject core node guide line don't mention a lot about the API for the inject node, so i thought i'd supply it.
i really needed this information at this specific spot, and i had to look long to find it elsewhere, hope this helps out others who also wonder about the inject node  API.

Author: robskaar

docs/user-guide/nodes.md

@@ -36,8 +36,12 @@ The `payload` can be set to a variety of different types:
  - a String, number, boolean, Buffer or Object
  - a Timestamp in milliseconds since January 1st, 1970
 
-*Since Node-RED 1.1.0*, the Inject node can now set any property on the message.
+The `interval` can maximum be set to every 596 hours (equivalent of about 24 days).
+If you are looking at intervals greater than one day - consider using a scheduler node that can cope with power outages and restarts.
+
+The `interval between tmes` and `at a specific time` options use the standard cron system. This means that 20 minutes will be at the next hour, 20 minutes past and 40 minutes past - not in 20 minutes time. If you want every 20 minutes from now - use the `interval` option.
 
+*Since Node-RED 1.1.0*, the Inject node can now set any property on the message.
 ***
 
 <img alt="Debug node" style="float: right; margin-top: 20px;" src="/docs/user-guide/images/node_debug.png" width="169px">