kdump: Document kernel data exported in the vmcoreinfo note

Document data exported in vmcoreinfo and briefly describe its use by
userspace tools.

 [ bp: heavily massage and redact the text. ]

Suggested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: anderson@redhat.com
Cc: k-hagio@ab.jp.nec.com
Cc: kexec@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: mingo@redhat.com
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190110121944.6050-2-lijiang@redhat.com