Adding notes about ShapeBuilder changes to migration doc.

docs/reference/migration/migrate_3_0.asciidoc

@@ -430,6 +430,10 @@ For simplicity, only one way of adding the ids to the existing list (empty by de
 error description). This will influence code that use the `IndexRequest.opType()` or `IndexRequest.create()`
 to index a document only if it doesn't already exist.
 
+==== ShapeBuilders
+
+`InternalLineStringBuilder` is removed in favour of `LineStringBuilder`, `InternalPolygonBuilder` in favour of PolygonBuilder` and `Ring` has been replaced with `LineStringBuilder`. Also the abstract base classes `BaseLineStringBuilder` and `BasePolygonBuilder` haven been merged with their corresponding implementations.
+
 [[breaking_30_cache_concurrency]]
 === Cache concurrency level settings removed