Documentation: irq/concepts: Minor improvements

Just note in the docs:

 1) A PCI device as an example for shared interrupts
 2) A sparse tree can be used for interrupts too
 3) i8259s which have 8 pins

[ tglx: Fix up subject prefix ]

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250319092951.37667-55-jirislaby@kernel.org