Input: adxl34x - documentation cleanup

This patch clarifies a few bits of documentation in the header file
for the adxl34x driver.

Signed-off-by: Michael Tandy <lkml@mkt.me.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>