items.md: override labels (openhab#738)

* items.md: override labels

Added information about how to override label text and state representation on sitemaps.

Signed-off-by: Josua Arndt <josuaarndt@live.de>

* Typos

* Add item override example

Signed-off-by: Josua Arndt <josuaarndt@live.de>

* Added UoM link

Signed-off-by: Josua Arndt <josuaarndt@live.de>

* Update code fences and example comments

Signed-off-by: Jerome Luckenbach <github@luckenba.ch>

Author: Josar

configuration/items.md

@@ -208,12 +208,14 @@ Graphical UIs will display the label text when the Item is included, e.g. in [Ba
 Some I/O services (e.g. the Amazon Alexa skill) also use the label to match an external voice command to an Item.
 
 In textual configurations the label, in quotation marks, appears next to the optional state presentation field in square brackets (see below).
-The label for the Item in the following example is "Temperature":
+The label for the Item in the following example is "Temperature and the optional state representation is set to be displayed, e.g. as "23.9 °C":
 
 ```java
 Number Livingroom_Temperature "Temperature [%.1f °C]"
 ```
 
+Channel labels can be overwritten by Item definitions and Item labels can be overwritten in [Sitemaps]({{base}}/configuration/sitemaps.html#element-types).
+
 {: #state}
 ### State
 

configuration/sitemaps.md

@@ -152,6 +152,25 @@ This provides the flexibility to present Items in the way desired in your home a
 If no label or icon are specified in the Sitemap, then the label and/or icon you assigned to the Item will be displayed.
 Setting a value for `label` or `icon` of a Sitemap element will override the values defined for the linked Item.
 
+It has to be considered that if the label defined in a Channel or an Item contains text and state, these representations have to be overwritten separately in the Sitemap.
+In the following example a Item which has a label and state defined is overwritten.
+
+```java
+sitemap demo label="My home automation" {
+    Frame label="Temperature" {
+        // Overrides only the text, but will keep the state format from the Item definition
+        Text item=Livingroom_Temperature label="Livingroom"
+        // Overrides the text and hides any state representation.
+        Text item=Livingroom_Temperature label="Livingroom []"
+        // Overrides the text and state representation
+        // and also changes the state unit to Fahrenheit
+        // if the value of the item supports UoM (link below) the value will be transformed.
+        Text item=Livingroom_Temperature label="Livingroom [%.2f °F]"
+    }
+}
+```
+UoM = [Units of Measurment]({{base}}/concepts/units-of-measurement.html)
+
 -   Additional parameters such as `mappings` and `valuecolor` are described below.
 
 ### Element Type 'Frame'