Updates Logstash pipeline ID description to point out the restrictions/acceptance. (elastic#135557)

(cherry picked from commit 83072b0)

Author: mashhurs

docs/reference/rest-api/logstash/put-pipeline.asciidoc

@@ -36,7 +36,9 @@ replaced.
 ==== {api-path-parms-title}
 
 `<pipeline_id>`::
-  (Required, string) Identifier for the pipeline.
+(Required, string)
+Identifier for the pipeline.
+It must begin with a letter or underscore and can contain only letters, underscores, dashes, hyphens, and numbers.
 
 [[logstash-api-put-pipeline-request-body]]
 ==== {api-request-body-title}