Document the doubletap_filter sysfs attribute for ThinkPad ACPI driver.

Signed-off-by: Vishnu Sankar <vishnuocv@gmail.com>
---
 .../admin-guide/laptops/thinkpad-acpi.rst     | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/admin-guide/laptops/thinkpad-acpi.rst b/Documentation/admin-guide/laptops/thinkpad-acpi.rst
index 4ab0fef7d440..a1e84d25e151 100644
--- a/Documentation/admin-guide/laptops/thinkpad-acpi.rst
+++ b/Documentation/admin-guide/laptops/thinkpad-acpi.rst
@@ -1521,6 +1521,26 @@ Currently 2 antenna types are supported as mentioned below:
 The property is read-only. If the platform doesn't have support the sysfs
 class is not created.
 
+doubletap_filter
+----------------
+
+sysfs: doubletap_filter
+
+Controls whether TrackPoint doubletap events are filtered out. Doubletap is a
+feature where quickly tapping the TrackPoint twice triggers a special function key event.
+
+The available commands are::
+
+                cat /sys/devices/platform/thinkpad_acpi/doubletap_filter
+                echo 1 | sudo tee /sys/devices/platform/thinkpad_acpi/doubletap_filter
+                echo 0 | sudo tee /sys/devices/platform/thinkpad_acpi/doubletap_filter
+
+Values:
+	* 0 - doubletap events are processed (default)
+	* 1 - doubletap events are filtered out (ignored)
+
+	This setting can also be toggled via the Fn+doubletap hotkey.
+
 Auxmac
 ------
 
-- 
2.51.0