Document thread-safety for ingest processors

With this commit we document that ingest processors need to be
thread-safe. Previously this could be inferred from reading the
source code but we got several user questions about this so
it is stated explicitly in the Javadocs of Processor now.

core/src/main/java/org/elasticsearch/ingest/Processor.java

@@ -29,6 +29,8 @@ import java.util.Map;
 /**
  * A processor implementation may modify the data belonging to a document.
  * Whether changes are made and what exactly is modified is up to the implementation.
+ *
+ * Processors may get called concurrently and thus need to be thread-safe.
  */
 public interface Processor {