DATAMONGO-2623 - Add support for $function and $accumulator aggregation operators.

Original pull request: #887.

src/main/asciidoc/new-features.adoc

@@ -8,6 +8,7 @@
 * Reactive SpEL support in `@Query` and `@Aggregation` query methods.
 * Aggregation hints via `AggregationOptions.builder().hint(bson).build()`.
 * Extension Function `KProperty.asPath()` to render property references into a property path representation.
+* Server-side JavaScript aggregation expressions `$function` and `$accumulator` via `ScriptOperators`.
 
 [[new-features.3.0]]
 == What's New in Spring Data MongoDB 3.0

src/main/asciidoc/reference/mongodb.adoc

@@ -2559,6 +2559,7 @@ The MongoDB Aggregation Framework provides the following types of aggregation op
 * Lookup Aggregation Operators
 * Convert Aggregation Operators
 * Object Aggregation Operators
+* Script Aggregation Operators
 
 At the time of this writing, we provide support for the following Aggregation Operations in Spring Data MongoDB:
 
@@ -2606,6 +2607,9 @@ At the time of this writing, we provide support for the following Aggregation Op
 
 | Object Aggregation Operators
 | `objectToArray`, `mergeObjects`
+
+| Script Aggregation Operators
+| `function`, `accumulator`
 |===
 
 * The operation is mapped or added by Spring Data MongoDB.