Document new vm plugin options

doc/manual/modules/performance/ref_available-tuned-plug-ins.adoc

@@ -56,7 +56,13 @@ Sets autosuspend timeout of USB devices to the value specified by the [option]`a
 The value `0` means that autosuspend is disabled.
 
 `vm`::
-Enables or disables transparent huge pages depending on the Boolean value of the [option]`transparent_hugepages` option.
+Tunes selected sysctl options in `/proc/sys/vm`, currently
+[option]`dirty_ratio`, [option]`dirty_background_ratio`,
+[option]`dirty_bytes`, and [option]`dirty_background_bytes`.
+For details of these options, see link:https://docs.kernel.org/admin-guide/sysctl/vm.html[docs.kernel.org].
++
+Additionaly enables or disables transparent huge pages depending on
+the value of the [option]`transparent_hugepages` option.
 +
 Valid values of the [option]`transparent_hugepages` option are:
 +

tuned/plugins/plugin_vm.py

@@ -15,9 +15,15 @@ class VMPlugin(base.Plugin):
 	"""
 	`vm`::
 	
-	Enables or disables transparent huge pages depending on value of the
-	[option]`transparent_hugepages` option. The option can have one of three
-	possible values `always`, `madvise` and `never`.
+	Tunes selected sysctl options in `/proc/sys/vm`, currently
+	[option]`dirty_ratio`, [option]`dirty_background_ratio`,
+	[option]`dirty_bytes`, and [option]`dirty_background_bytes`.
+	See https://docs.kernel.org/admin-guide/sysctl/vm.html for detailed
+	documentation of these options.
+	+
+	Additionaly enables or disables transparent huge pages depending on
+	the value of the [option]`transparent_hugepages` option. The option
+	can have one of three possible values: `always`, `madvise` and `never`.
 	+
 	.Disable transparent hugepages
 	====