Documentation: core-api: entry: Replace deprecated KVM entry/exit functions

Documentation: core-api: entry: Replace deprecated KVM entry/exit functions

The x86-specific functions kvm_guest_{enter,exit}_irqoff() were removed
and replaced by the generic guest_state_{enter,exit}_irqoff() in commit
ef9989afda73 ("kvm: add guest_state_{enter,exit}_irqoff()") and commit
b2d2af7e5df3 ("kvm/x86: rework guest entry logic").

Update the references in the entry/exit handling documentation.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Nicolas Saenz Julienne <nsaenz@amazon.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250723075134.105132-1-ajd@linux.ibm.com