Documentation: kdump: clean up the outdated description

After commit 443cbaf9 ("crash: split vmcoreinfo exporting code out
from crash_core.c"), Kconfig item CRASH_CORE has gone away in kernel. 
Items VMCORE_INFO and CRASH_RESERVE are used instead.

So clean up the outdated description about CRASH_CORE and update it
accordingly.

Link: https://lkml.kernel.org/r/20240329132825.1102459-3-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>