[DOCS] Clarify that pipeline metadata IS required (#81184)

Author: debadair

x-pack/docs/en/rest-api/logstash/put-pipeline.asciidoc

@@ -52,7 +52,7 @@ documentation].
 
 `pipeline_metadata`::
 (Required, object)
-Optional metadata about the pipeline. May have any contents. This metadata is
+Arbitrary metadata about the pipeline. May have any contents. This metadata is
 not generated or used by {es} or {ls}.
 
 `pipeline_settings`::