[DOCS] Clarified watcher cleaner service behaviour (elastic/x-pack-elasticsearch#3135)

* [DOCS] Clarified watcher cleaner service behaviour

* [DOCS] Fixed typo in watcher settings

* [DOC] Clarified watcher and monitor setting dependencies

Original commit: elastic/x-pack-elasticsearch@1386139460

docs/en/settings/monitoring-settings.asciidoc

@@ -72,15 +72,22 @@ Settings API.
 
 `xpack.monitoring.history.duration`::
 
-Sets the retention duration beyond which the indices created by a Monitoring exporter will
-be automatically deleted. Defaults to `7d` (7 days).
-+
-This setting has a minimum value of `1d` (1 day) to ensure that something is being monitored,
-and it cannot be disabled.
+Sets the retention duration beyond which the indices created by a Monitoring
+exporter are automatically deleted. Defaults to `7d` (7 days).
 +
+--
+This setting has a minimum value of `1d` (1 day) to ensure that something is
+being monitored, and it cannot be disabled.
+
 IMPORTANT: This setting currently only impacts `local`-type exporters. Indices created using
 the `http` exporter will not be deleted automatically.
 
+If both {monitoring} and {watcher} are enabled, you can use this setting to
+affect the {watcher} cleaner service too. For more information, see the
+`xpack.watcher.history.cleaner_service.enabled` setting in the 
+<<notification-settings>>.
+--
+
 `xpack.monitoring.exporters`::
 
 Configures where the agent stores monitoring data. By default, the agent uses a

docs/en/settings/notification-settings.asciidoc

@@ -35,9 +35,13 @@ required. For more information, see
 {xpack-ref}/encrypting-data.html[Encrypting sensitive data in {watcher}].
 
 `xpack.watcher.history.cleaner_service.enabled`::
-Set to `false` (default) to disable the cleaner service, which removes previous
-versions of {watcher} indices (for example, .watcher-history*) when it
-determines that they are old.
+Set to `false` (default) to disable the cleaner service. If this setting is
+`true`, the `xpack.monitoring.enabled` setting must also be set to `true`. The
+cleaner service removes previous versions of {watcher} indices (for example,
+`.watcher-history*`) when it determines that they are old. The duration of
+{watcher} indices is determined by the `xpack.monitoring.history.duration`
+setting, which defaults to 7 days. For more information about that setting,
+see <<monitoring-settings>>.
 
 `xpack.http.proxy.host`::
 Specifies the address of the proxy server to use to connect to HTTP services.