[DOCS] Metricbeat: Document unavailable and unknown as `filesyste…

…m.ignore_types` values (#36946)

Documents `unavailable` and `unknown` as valid values for the `filesystem.ignore_types` array. Also adds a related example.

Closes #35187.

metricbeat/module/system/filesystem/_meta/docs.asciidoc

@@ -12,20 +12,28 @@ This metricset is available on:
 [float]
 === Configuration
 
-*`filesystem.ignore_types`* - A list of filesystem types to ignore. Metrics will
-not be collected from filesystems matching these types. This setting also
-affects the `fsstats` metricset. If this option is not set, metricbeat ignores
-all types for virtual devices in systems where this information is available (e.g.
-all types marked as `nodev` in `/proc/filesystems` in Linux systems). This can be set to an empty list (`[]`)
-to make metricbeat report all filesystems, regardless of type.
+*`filesystem.ignore_types`* - An array of filesystem types to ignore. Metrics
+will not be collected from filesystems matching these types. If this option is
+not set, {metricbeat} ignores all types for virtual devices in systems where this
+information is available (e.g. all types marked as `nodev` in
+`/proc/filesystems` in Linux systems). This setting affects the `fsstats`
+metricset.
+
+To have {metricbeat} report on all filesystems, regardless of type, set
+`filesystem.ignore_types` to an empty array (`[]`).
+
+
+To ignore unavailable volumes, such as CD-ROM drives, on Windows include
+`unavailable` as a value in the array. To ignore unknown filesystems on Windows,
+include `unknown` as a value in the array.
 
 [float]
 === Filtering
 
-Often there are mounted filesystems that you do not want Metricbeat to report
-metrics on. One option is to configure Metricbeat to ignore specific filesystem
+There may be mounted filesystems that you don't want {metricbeat} to report
+metrics on. One option is to configure {metricbeat} to ignore specific filesystem
 types. This can be accomplished by configuring `filesystem.ignore_types` with
-a list of filesystem types to ignore. In this example we are ignoring three
+an array of filesystem types to ignore. In this example we are ignoring three
 types of filesystems.
 
 [source,yaml]
@@ -37,10 +45,21 @@ metricbeat.modules:
     filesystem.ignore_types: [nfs, smbfs, autofs]
 ----
 
+A common approach is to ignore any `unavailable` or `unknown` filesystems on Windows. For example:
+
+[source,yaml]
+----
+metricbeat.modules:
+  - module: system
+    period: 30s
+    metricsets: ["filesystem"]
+    filesystem.ignore_types: [unavailable, unknown]
+----
+
 Another strategy to deal with these filesystems is to configure a `drop_event`
 processor that matches the `mount_point` using a regular expression. This type
 of filtering occurs after the data has been collected so it can be less
-efficient than the previous method.
+efficient than specifying `filesystem.ignore_types`.
 
 [source,yaml]
 ----