Update preference.asciidoc

Clarify that `preference` is a query string parameter only
and provide an example.

docs/reference/search/request/preference.asciidoc

@@ -5,7 +5,7 @@ Controls a `preference` of which shard replicas to execute the search
 request on. By default, the operation is randomized between the shard
 replicas.
 
-The `preference` can be set to:
+The `preference` is a query string parameter which can be set to:
 
 [horizontal]
 `_primary`:: 
@@ -40,3 +40,20 @@ Custom (string) value::
 	states. A sample value can be something like the web session id, or the
 	user name.
 
+For instance, use the user's session ID to ensure consistent ordering of results
+for the user:
+
+[source,js]
+------------------------------------------------
+curl localhost:9200/_search?preference=xyzabc123 -d '
+{
+    "query": {
+        "match": {
+            "title": "elasticsearch"
+        }
+    }
+}
+'
+------------------------------------------------
+
+