Documentation: Improve crash_kexec_post_notifiers description

The crash_kexec_post_notifiers description could be improved a bit,
by clarifying its upsides (yes, there are some!) and be more descriptive
about the downsides, specially mentioning code that enables the option
unconditionally, like Hyper-V[0], PowerPC (fadump)[1] and more recently,
AMD SEV-SNP[2].

[0] Commit a1158956 ("x86/Hyper-V: Report crash register data or kmsg before running crash kernel").
[1] Commit 06e629c2 ("powerpc/fadump: Fix inaccurate CPU state info in vmcore generated with panic").
[2] Commit 8ef97958 ("crypto: ccp: Add panic notifier for SEV/SNP firmware shutdown on kdump").

Reviewed-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241027204159.985163-1-gpiccoli@igalia.com