Documentation: kdump: clean up the outdated description

After commit 443cbaf9e2fd ("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So clean up the outdated description about CRASH_CORE and update it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/[e-mail]
Signed-off-by: Baoquan He <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Huacai Chen <[e-mail]>
Cc: WANG Xuerui <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/kdump/kdump.rst

···
 
 	CONFIG_KEXEC_CORE=y
 
-   Subsequently, CRASH_CORE is selected by KEXEC_CORE::
-
-	CONFIG_CRASH_CORE=y
-
 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
    filesystems." This is usually enabled by default::
 
···
    features"::
 
 	CONFIG_CRASH_DUMP=y
+
+   And this will select VMCORE_INFO and CRASH_RESERVE::
+	CONFIG_VMCORE_INFO=y
+	CONFIG_CRASH_RESERVE=y
 
 2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::