printk: nbcon: Clarify rules of the owner/waiter matching

printk: nbcon: Clarify rules of the owner/waiter matching

The functions nbcon_owner_matches() and nbcon_waiter_matches()
use a minimal set of data to determine if a context matches.
The existing kerneldoc and comments were not clear enough and
caused the printk folks to re-prove that the functions are
indeed reliable in all cases.

Update and expand the explanations so that it is clear that the
implementations are sufficient for all cases.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-6-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>