irqdomain: update documentation

This updates the IRQdomain documentation a bit, by adding a more
verbose explanation to why we need this, and by adding some
extended documentation of the irq_domain_simple() usecase.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>