Bluetooth: GATT: Doc: start_handle is updated when reading by type

At `gatt.c:4560`, `params->start_handle` is updated just before calling
`params->func`. I think this is intentional, as the read handle is
useful information for the applicaion and there is no other method of
getting it. This change just documents this behavior.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>