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
7c321eb2b843 ("x86/kdump: Remove the backup region handling")
commit
6f599d84231f ("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>