Documentation: kdump: Remind user of nr_cpus

nr_cpus can help to save memory. So we should remind user of it.
Signed-off-by: NZhou Wenjian <zhouwj-fnst@cn.fujitsu.com>
Acked-by: NBaoquan He <bhe@redhat.com>
Acked-by: NXunlei Pang <xpang@redhat.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/kdump/kdump.txt

@@ -393,6 +393,8 @@ Notes on loading the dump-capture kernel:
 * We generally don' have to bring up a SMP kernel just to capture the
   dump. Hence generally it is useful either to build a UP dump-capture
   kernel or specify maxcpus=1 option while loading dump-capture kernel.
+  Note, though maxcpus always works, you had better replace it with
+  nr_cpus to save memory if supported by the current ARCH, such as x86.
 
 * For s390x there are two kdump modes: If a ELF header is specified with
   the elfcorehdr= kernel parameter, it is used by the kdump kernel as it