Documentation: livepatch: outline Elf format and requirements for patch modules

Document livepatch module requirements and the special Elf constants patch
modules use.

Signed-off-by: Jessica Yu <jeyu@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>