iio: Documentation: Add trigger name attribute ABI documentation

This patch adds an entry in ABI Documentation for the name attribute
issued when a trigger is created.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>