Documentation: kdump: update kdump guide

Some parts of the guide are aged, hence need be updated.

1) The backup area of the 1st 640K on X86_64 has been removed
   by below commits, update the description accordingly.

   commit 7c321eb2 ("x86/kdump: Remove the backup region handling")
   commit 6f599d84 ("x86/kdump: Always reserve the low 1M when the crashkernel option is specified")

2) Sort out the descripiton of "crashkernel syntax" part.

3) And some other minor cleanups.
Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Dave Young <dyoung@redhat.com>
Link: https://lore.kernel.org/r/20210609083218.GB591017@MiWiFi-R3L-srv
[jc: added blank line to fix added build warning]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>