docs: handling-regressions.rst: recommend using "Closes:" tags

Update the handling-regressions guide to recommend using "Closes:" tags
rather than "Link:" when referencing fixed reports. The latter was used
originally but now is only recommended when the given patch only fixes
part of the issue, as described in submitting-patches. Briefly mention
that and also note that regzbot currently doesn't make a distinction.

Also fix a typo.

Acked-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Karel Balej <balejk@matfyz.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240513084145.2460-1-balejk@matfyz.cz