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

Author: Jiri Slaby (SUSE)

Documentation/core-api/irq/concepts.rst

@@ -4,18 +4,20 @@ What is an IRQ?
 
 An IRQ is an interrupt request from a device. Currently, they can come
 in over a pin, or over a packet. Several devices may be connected to
-the same pin thus sharing an IRQ.
+the same pin thus sharing an IRQ. Such as on legacy PCI bus: All devices
+typically share 4 lanes/pins. Note that each device can request an
+interrupt on each of the lanes.
 
 An IRQ number is a kernel identifier used to talk about a hardware
 interrupt source. Typically, this is an index into the global irq_desc
-array, but except for what linux/interrupt.h implements, the details
-are architecture specific.
+array or sparse_irqs tree. But except for what linux/interrupt.h
+implements, the details are architecture specific.
 
 An IRQ number is an enumeration of the possible interrupt sources on a
 machine. Typically, what is enumerated is the number of input pins on
 all of the interrupt controllers in the system. In the case of ISA,
-what is enumerated are the 16 input pins on the two i8259 interrupt
-controllers.
+what is enumerated are the 8 input pins on each of the two i8259
+interrupt controllers.
 
 Architectures can assign additional meaning to the IRQ numbers, and
 are encouraged to in the case where there is any manual configuration