Input: elantech - update the documentation

Input: elantech - update the documentation

A chapter is added to describe the trackpoint packets.

A section is added to describe the behaviour of the knob crc_enabled in
sysfs.

The introduction of the documentation only mentioned v1/v2, but in the
last part it already contains explanation of v3 and v4. The introduction
is updated.

Signed-off-by: Ulrik De Bie <ulrik.debie-os@e2big.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>