Add filebeat notice

docs/user/monitoring/monitoring-troubleshooting.asciidoc

@@ -23,6 +23,23 @@ You cannot monitor a version 6.3 or later cluster from {kib} version 6.2 or earl
 To resolve this issue, upgrade {kib} to 6.3 or later. See 
 {stack-ref}/upgrading-elastic-stack.html[Upgrading the {stack}]. 
 
+[float]
+=== {filebeat} index is corrupt
+
+*Symptoms:*
+
+The *Stack Monitoring* application displays a Monitoring Request error indicating
+that an illegal argument exception has occurred because fielddata is disabled on
+text fields by default.
+
+*Resolution*
+
+ . Stop all your {filebeat} instances.
+ . Delete indices beginning with `filebeat-$VERSION`, where `VERSION` corresponds
+   to the version of {filebeat} running.
+. Restart all your {filebeat} instances.
+
+
 [float]
 === No monitoring data is visible in {kib}