Painless Context Doc: Add filter context example

docs/painless/painless-contexts/painless-filter-context.asciidoc

@@ -23,4 +23,43 @@ query to include and exclude documents.
 
 *API*
 
-The standard <<painless-api-reference, Painless API>> is available.
+The standard <<painless-api-reference, Painless API>> is available.
+
+*Example*
+
+To run this example, first follow the steps in
+<<painless-context-examples, context examples>>.
+
+This script finds all unsold documents that cost less than $18.
+
+[source,Painless]
+----
+doc['sold'].value == false && doc['cost'].value < 18
+----
+
+Defining cost as a script parameter enables the cost to be configured
+in the script query request. For example, the following request finds
+all available theatre seats for evening performances that are under $18.
+
+[source,js]
+----
+GET evening/_search
+{
+    "query": {
+        "bool" : {
+            "filter" : {
+                "script" : {
+                    "script" : {
+                        "source" : "doc['sold'].value == false && doc['cost'].value < params.cost",
+                        "params" : {
+                            "cost" : 18
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+----
+// CONSOLE
+// TEST[skip: requires setup from other pages]

modules/lang-painless/src/test/java/org/elasticsearch/painless/ContextExampleTests.java

@@ -1,3 +1,4 @@
+
 /*
  * Licensed to Elasticsearch under one or more contributor
  * license agreements. See the NOTICE file distributed with
@@ -359,5 +360,41 @@ public void testScriptFieldsScript() {
             singletonMap("_source", source), true)
         );
     }
+
+    // Use script query request to filter documents
+    /*
+    GET localhost:9200/evening/_search
+    {
+        "query": {
+            "bool" : {
+                "filter" : {
+                    "script" : {
+                        "script" : {
+                            "source" : "doc['sold'].value == false && doc['cost'].value < params.cost",
+                            "params" : {
+                                "cost" : 18
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+    */
+
+    public void testFilterScript() {
+        Map<String, Object> source = new HashMap<>();
+        source.put("sold", false);
+        source.put("cost", 15);
+
+        Map<String, Object> params = new HashMap<>();
+        params.put("_source", source);
+        params.put("cost", 18);
+
+        boolean result = (boolean) exec(
+            " params['_source']['sold'] == false && params['_source']['cost'] < params.cost;",
+            params, true);
+        assertTrue(result);
+    }
 }