Documentation/IRQ-domain.txt: Document irq_domain_create_{linear, tree}

They have the same functionalities as irq_domain_add_{linear, tree},
except fro accepting different first argument.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>