output.logstash.index doc update (elastic#17496) (elastic#17810)

Updated the output.logstash.index section to include a note explaing that this parameter's value will be assigend to the metadata.beat field in order to be reused in the Logstash output section.

Co-authored-by: chrousto <christophe.chazeau@gmail.com>

Co-authored-by: chrousto <christophe.chazeau@gmail.com>

libbeat/outputs/logstash/docs/logstash.asciidoc

@@ -325,6 +325,9 @@ The index root name to write events to. The default is the Beat name. For
 example +"{beat_default_index_prefix}"+ generates +"[{beat_default_index_prefix}-]{version}-YYYY.MM.DD"+
 indices (for example, +"{beat_default_index_prefix}-{version}-2017.04.26"+).
 
+NOTE: This parameter's value will be assigned to the `metadata.beat` field. It
+can then be accessed in Logstash's output section as `%{[@metadata][beat]}`.
+
 ===== `ssl`
 
 Configuration options for SSL parameters like the root CA for Logstash connections. See