added expire documentation (openhab#1293)

Signed-off-by: Kai Kreuzer <kai@openhab.org>

configuration/items.md

@@ -658,6 +658,7 @@ The first example shows a symbiosis of the network health Binding and the Wake-o
 The second example shows a common use case for the [Expire Binding](/addons/bindings/expire1/)
 where the mysensors Binding will update temperature readings regularly but the expire Binding will also listen and eventually modify the Item state.
 
+{: #autoupdate}
 #### Parameter `autoupdate`
 
 `autoupdate="false"` is a special instruction which keeps the current state of the item, even if a *command* has been received.
@@ -669,6 +670,40 @@ Example:
 Switch Garage_Gate {binding="xxx", autoupdate="false"}
 ```
 
+{: #expire}
+#### Parameter `expire`
+
+This parameter allows to post an update or command to an item after a period of time has passed.
+
+The expiration timer is started or restarted every time an item receives an update or a command *other than* the specified "expire" update/command.
+Any future expiring update or command is cancelled, if the item receives an update or command that matches the "expire" update/command.
+
+The parameter accepts a duration of time that can be a combination of hours, minutes and seconds in the format
+
+```
+expire="1h 30m 45s"
+expire="1h05s"
+expire="55h 59m 12s"
+```
+
+Every part is optional, but all parts present must be in the given order (hours, minutes, seconds).
+Whitespaces are allowed between the sections.
+
+This duration can optionally be followed by a comma and the state or command to post, when the timer expires.
+If this optional section is not present, it defaults to the Undefined (`UnDefType.UNDEF`) state.
+
+```
+Player MyPlayer   { expire="1h,command=STOP" } // send STOP command after one hour
+Number MyChannel  { expire="5m,state=0" }      // update state to 0 after five minutes
+String MyMessage  { expire="3m12s,Hello" }     // update state to Hello after three minutes and 12 seconds
+Switch MySwitch   { expire="2h" }              // update state to Undefined two hours after last value
+```
+
+Note that the `state=` part is optional.
+
+In the special case of a String item, it is possible to define a state/command as the string "UNDEF" or "NULL" by putting it into single quotes (e.g. "1m,state='UNDEF'").
+Without the quotes, the state would be the system type `UNDEF`.
+
 #### Profiles
 
 With Profiles, you're able to change the behavior how Channels interact with your Items. You can use *State Profiles* on State Channels and *Trigger Profiles* on Trigger Channels.