genirq: Fix percpu_devid irq affinity documentation

Stephen points out that some of the percpu_devid irq affinity
documentation is either missing or not matching the data structures.

Address all the issues in one go.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 87b0031f7f73 ("irqdomain: Add firmware info reporting interface")
Fixes: 258e7d28a3dc ("genirq: Add affinity to percpu_devid interrupt requests")
Signed-off-by: Marc Zyngier <maz@kernel.org>