remove `+` references for xpack.monitoring.collection.indices (#37282)

`+` for index name inclusions is no longer supported for 6.x+. This
commit removes references of the `+` from the documenation. System
indices additional example is also included.

fixes #37237

docs/reference/monitoring/collecting-monitoring-data.asciidoc

@@ -88,9 +88,11 @@ example:
 xpack.monitoring.collection.indices: logstash-*, index1, test2
 ----------------------------------
 
-You can prepend `+` or `-` to explicitly include or exclude index names or 
+You can prepend `-` to explicitly exclude index names or
 patterns. For example, to include all indices that start with `test` except 
-`test3`, you could specify `+test*,-test3`.
+`test3`, you could specify `test*,-test3`. To include system indices such as
+.security and .kibana, add `.*` to the list of included names.
+For example `.*,test*,-test3`
 --
 
 .. Optional: Specify how often to collect monitoring data. The default value for 

docs/reference/settings/monitoring-settings.asciidoc

@@ -76,9 +76,10 @@ Sets the timeout for collecting the cluster statistics. Defaults to `10s`.
 
 Controls which indices Monitoring collects data from. Defaults to all indices. Specify the index names
 as a comma-separated list, for example `test1,test2,test3`. Names can include wildcards, for
-example `test*`. You can explicitly include or exclude indices by prepending
-`+` to include the index, or `-` to exclude the index. For example, to include all indices that
-start with `test` except `test3`, you could specify `+test*,-test3`.
+example `test*`. You can explicitly exclude indices by prepending `-`. For example `test*,-test3` will
+monitor all indexes that start with `test` except for `test3`. System indices like .security* or .kibana*
+always start with a `.`, and generally should be monitored. Consider adding `.*` to the list of indices
+ensure monitoring of system indices. For example `.*,test*,-test3`
 
 `xpack.monitoring.collection.index.stats.timeout`::