kexec_core: Add and update comments regarding the KEXEC_JUMP flow

The KEXEC_JUMP flow is analogous to hibernation flows occurring before
and after creating an image and before and after jumping from the
restore kernel to the image one, which is why it uses the same device
callbacks as those hibernation flows.

Add comments explaining that to the code in question and update an
existing comment in it which appears a bit out of context.

No functional changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250109140757.2841269-8-dwmw2@infradead.org