Documentation: kdump: clean up the outdated description

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So clean up the outdated description about CRASH_CORE and update it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

Author: Baoquan He

Documentation/admin-guide/kdump/kdump.rst

@@ -136,10 +136,6 @@ System kernel config options
 
 	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::
 
@@ -168,6 +164,10 @@ Dump-capture kernel config options (Arch Independent)
 
 	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"::
 
 	CONFIG_PROC_VMCORE=y