Documentation: livepatch: add section about arch-specific code

Document usage of arch-specific elf sections in livepatch as well
as implementation of arch-specific code.

[jkosina@suse.cz: fix wording as suggested by Petr Mladek]
Signed-off-by: Jessica Yu <jeyu@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>