hwdb: Document how to properly set the mount-matrix for the base-accelerometer in 360 degree hinges style 2-in-1s with 2 accelerometers

Document how the mount-matrix for the base-accelerometer must be set on
360 degree hinges style 2-in-1s with 2 sensors (one in the display and
1 in the base).

Note the choice to define the lid being fully closed as an angle of
0 degrees is based on the ACPI tables of devices with a BOSC0200
ACPI device-node describing both sensors. In this case the ACPI
tables contain mount-matrix info (and the kernel will soon support
reading this and exporting it to userspace) and the mount-matrices
defined in these ACPI tables are such that the angle of the G-force
vector measured by the sensors is identical for both sensors when
the laptop's lid is fully closed.

This also feels more natural then defining the laptop being fully
open (180 degrees open) as the home / 0 degree angle position.

hwdb.d/60-sensor.hwdb

@@ -67,6 +67,16 @@
 # or 'display'. The default, when unset, is equivalent to:
 #    ACCEL_LOCATION=display
 #
+# A note about setting ACCEL_MOUNT_MATRIX for ACCEL_LOCATION=base sensors,
+# on 360 degree hinges style 2-in-1s with 2 sensors (one in the display and
+# 1 in the base). Userspace will use both sensors to calculate the angle between
+# the 2 halves and the angle is defined as being 0 when the device is folded
+# as a regular clamshell laptop with its lid closed. This means that the
+# base-accelerometer's mount-matrix must be such, that after applying
+# the mount-matrices to both sensors, the base-accelerometer's readings must
+# be identical to the display-accelerometer's readings (when the lid is
+# closed).
+#
 # Sort by brand, model
 
 #########################################