docs: iio: adxl345: update event attributes and scaling math

docs: iio: adxl345: update event attributes and scaling math

Update the documentation to reflect the recent driver additions for
event scaling and the double tap threshold value, alongside correcting
existing technical errors in scale calculations.

Key changes:
- Fix the 62.5 g/LSB typo to 62.5 mg/LSB and add SI unit conversion.
- Correct decimal precision of in_accel_scale and
  in_accel_scale_available to match the actual SI unit (m/s^2)
  values reported by the driver.
- Document the newly generated event scale attributes in the ABI table
  (e.g., in_accel_mag_rising_scale, in_accel_gesture_singletap_scale).
- Document the newly exposed in_accel_gesture_doubletap_value attribute.
- Add a sysfs example showing how to read and interpret the newly
  implemented event scale factor.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Taha Ed-Dafili <0rayn.dev@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>