Update service documentation to add target and selector info (#819)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

docs/dev_101_services.md

@@ -59,20 +59,39 @@ Services are published under the domain name of your integration, so in `service
 ```yaml
 # Example services.yaml entry
 
+# Service ID
 set_speed:
+  # Service name as shown in UI
+  name: Set speed
   # Description of the service
   description: Sets fan speed.
+  # If the service accepts entity IDs, target allows the user to specify entities by entity, device, or area. If `target` is specified, `entity_id` should not be defined in the `fields` map. By default it shows only targets matching entities from the same domain as the service, but if further customization is required, target supports the entity, device, and area selectors (https://www.home-assistant.io/docs/blueprint/selectors/). Entity selector parameters will automatically be applied to device and area, and device selector parameters will automatically be applied to area. 
+  target:
   # Different fields that your service accepts
   fields:
     # Key of the field
-    entity_id:
-      # Description of the field
-      description: Name(s) of the entities to set
-      # Example value that can be passed for this field
-      example: "fan.living_room"
     speed:
+      # Field name as shown in UI
+      name: Speed
+      # Description of the field
       description: Speed setting
+      # Whether or not field is required
+      required: true
+      # Advanced options are only shown when the advanced mode is enabled for the user
+      advanced: true
+      # Example value that can be passed for this field
       example: "low"
+      # The default value
+      default: "high"
+      # Selector (https://www.home-assistant.io/docs/blueprint/selectors/) to control the input UI for this field
+      selector:
+        select:
+          options:
+            - "off"
+            - "low"
+            - "medium"
+            - "high"
+
 ```
 
 ## Entity Services