Update script-fields-api.md (elastic#2209)

Add the MUCH needed example of using the `set` call to manipulate field
data.

---------

Co-authored-by: István Zoltán Szabó <[email protected]>

Author: philippkahr

explore-analyze/scripting/script-fields-api.md

@@ -25,6 +25,12 @@ Use the `field` API to access document fields:
 field('my_field').get(<default_value>)
 ```
 
+Alternatively use the shortcut of `$` to get a field.
+
+```painless
+$('my_field', <default_value>)
+```
+
 This API fundamentally changes how you access documents in Painless. Previously, you had to access the `doc` map with the field name that you wanted to access:
 
 ```painless
@@ -77,7 +83,6 @@ ZonedDateTime end = field('end').get(null);
 return start == null || end == null ? -1 : ChronoUnit.MILLIS.between(start, end)
 ```
 
-
 ## Supported mapped field types [_supported_mapped_field_types] 
 
 The following table indicates the mapped field types that the `field` API supports. For each supported type, values are listed that are returned by the `field` API (from the `get` and `as<Type>` methods) and the `doc` map (from the `getValue` and `get` methods).
@@ -112,3 +117,20 @@ The `fields` API currently doesn’t support some fields, but you can still acce
 | `wildcard` | `String` | - | `String` | `String` |
 | `flattened` | `String` | - | `String` | `String` |
 
+## Manipulation of the fields data
+
+The field API provides a `set(<value>)` operation that will take the field name and create the necessary structure. Calling this inside an ingest pipelines script processor context:
+
+```painless
+field("foo.bar").set("abc")
+```
+
+leads to the generation of this JSON representation.
+
+```json
+{
+  "foo": {
+    "bar": "abc"
+  }
+}
+```