irqdomain: Fix kernel-doc and add it to Documentation

irqdomain.c's kernel-doc exists, but is not plugged into Documentation/
yet.

Before plugging it in, fix it first: irq_domain_get_irq_data() and
irq_domain_set_info() were documented twice. Identically, by both
definitions for CONFIG_IRQ_DOMAIN_HIERARCHY and !CONFIG_IRQ_DOMAIN_HIERARCHY.

Therefore, switch the second kernel-doc into an ordinary comment -- change
"/**" to simple "/*". This avoids sphinx's: WARNING: Duplicate C
declaration

Next, in commit b7b37733 ("irqdomain: Fix the kernel-doc and plug it
into Documentation"), irqdomain.h's (header) kernel-doc was added into
core-api/genericirq.rst. But given the amount of irqdomain functions and
structures, move all these to core-api/irq/irq-domain.rst now.

Finally, add these newly fixed irqdomain.c's (source) docs there as
well.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250319092951.37667-58-jirislaby@kernel.org