Add syntax for multiple selector logging

libbeat/docs/loggingconfig.asciidoc

@@ -156,6 +156,12 @@ To see which selectors are available, run {beatname_uc} in debug mode
 after the log level and is enclosed in brackets.
 =====
 
+To configure multiple selectors, use the following syntax:
+["source","yaml",subs="attributes"]
+----
+logging.selectors: [ harvester, input ]
+----
+
 ifndef::serverless[]
 To override selectors at the command line, use the `-d` global flag (`-d` also
 sets the debug log level). For more information, see <<command-line-options>>.