add a rest test for this that seems to work, fix the documentation. thanks @s1monw

docs/reference/aggregations/bucket/terms-aggregation.asciidoc

@@ -443,7 +443,10 @@ Generating the terms using a script:
     "aggs" : {
         "genders" : {
             "terms" : {
-                "script" : "doc['gender'].value"
+                "script" : { 
+                    "inline": "doc['gender'].value"
+                    "lang": "painless"
+                }
             }
         }
     }
@@ -482,7 +485,10 @@ TIP: for indexed scripts replace the `file` parameter with an `id` parameter.
         "genders" : {
             "terms" : {
                 "field" : "gender",
-                "script" : "'Gender: ' +_value"
+                "script" : { 
+                    "inline" : "'Gender: ' +_value"
+                    "lang" : "painless" 
+                }
             }
         }
     }

modules/lang-painless/src/test/resources/rest-api-spec/test/plan_a/30_search.yaml

@@ -363,3 +363,33 @@
 
     - match: { hits.total: 1 }
     - match: { hits.hits.0.fields.foobar.0: 1 }
+
+---
+
+"Agg _value":
+    - do:
+        index:
+            index: test
+            type: test
+            id: 1
+            body: { "dummy_field": 1 }
+    - do:
+        indices.refresh: {}
+
+
+    - do:
+        index: test
+        search:
+            body:
+                aggs:
+                    value_agg:
+                        terms:
+                            field: dummy_field
+                            script:
+                                lang: painless
+                                inline: "_value + 1"
+
+    - match: { hits.total: 1 }
+    - match: { hits.hits.0._score: 1.0 }
+    - match: { aggregations.value_agg.buckets.0.key: 2 }
+    - match: { aggregations.value_agg.buckets.0.doc_count: 1 }