Add document about getting thing status in scripts and rule.

The change of the code is at eclipse-archived/smarthome#3001
After this commit, the end user can find the documentation about how to react
to thing status change in the rule.

Bug: openhab#517
Signed-off-by: Maoliang Huang <hellomao@outlook.com>

Incorporate more pull request feedback.

addons/actions.md

@@ -95,6 +95,29 @@ The Timer object supports the following methods:
 * `hasTerminated`: returns true if the code has run and completed
 * `reschedule(AbstractInstant instant)`: reschedules the timer to execute at the new time. If the Timer has terminated this method does nothing.
 
+### Thing Status Action
+
+`getThingStatusInfo(String thingUID)`: Gets status information of the given thing identified by `thingUID`.
+
+The result is of type `ThingStatusInfo`.
+It contains [Thing Status]({{base}}/concepts/things.html), [Status Details]({{base}}/concepts/things.html) and [Status Description]({{base}}/concepts/things.html).
+Refer to [Thing Status API]({{base}}/concepts/things.html) for how to get those information.
+If you just want to know the status, you can use `thingStatusInfo.getStatus().toString()` and the result will be one of the values in [Thing Status]({{base}}/concepts/things.html).
+
+> If the thing is removed or it's not added yet, it'll return null.
+
+For example:
+
+```java
+var thingStatusInfo = getThingStatusInfo("zwave:device:c5155aa4:node2")
+
+if ((thingStatusInfo != null) && (thingStatusInfo.getStatus().toString() == "ONLINE")) {
+    logInfo("ThingStatus", "The thing is online.")
+} else {
+    logError("ThingStatus", "The thing is offline or doesn't exist.")
+}
+```
+
 ## Installable Actions
 
 <table id="actions-select" class="striped">

configuration/rules-dsl.md

@@ -113,6 +113,7 @@ There are different categories of rule triggers:
 - **Item**(-Event)-based triggers: They react on events on the openHAB event bus, i.e. commands and status updates for items
 - **Time**-based triggers: They react at special times, e.g. at midnight, every hour, etc.
 - **System**-based triggers: They react on certain system statuses.
+- **Thing**-based triggers: They react on thing status, i.e. change from ONLINE to OFFLINE.
 
 Here are the details for each category:
 
@@ -178,6 +179,28 @@ then
 end
 ```
 
+### Thing Triggers
+
+You can listen for status updates or status changes (an update might leave the status unchanged).
+You can decide whether you want to catch only a specific status or any.
+Here is the syntax for all these cases (parts in square brackets are optional):
+
+```java
+Thing <thingUID> received update [<status>]
+Thing <thingUID> changed [from <status>] [to <status>]
+```
+
+The status used in the trigger and the script is a string (no quotes).
+You can find all the possible values for status from [Thing Status]({{base}}/concepts/things.html).
+And refer to [Thing Status Action]({{base}}/addons/actions.html) to find how to get thing status in the script.
+
+The `thingUID` is the identifier assigned to the Thing, manually in your configuration or automatically during auto discovery.
+You can find it from PaperUI or from Karaf remote console.
+For example, one z-wave device can be "zwave:device:c5155aa4:node14".
+
+> Note: You need to use quotes around `thingUID` if it contains special characters such as ':'.
+> The parser will not recognize the whole text correctly without quotes.
+
 ## Scripts
 
 The expression language used within scripts is the same that is used in the Xtend language - see the [documentation of expressions](http://www.eclipse.org/xtend/documentation/203_xtend_expressions.html) on the Xtend homepage.