[PATCH] kdump: add dmesg gdbmacro into document

Add gdb macro which print the kernel ring buffer into kdump docs

Signed-off-by: Akinobu Mita <mita@miraclelinux.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>