docs: submitting-patches: make section about the Link: tag more explicit

Mention the 'Link' tag in the section about adding URLs to the commit
msg, to make it clearer they "_primarily_ [...] should be about
background", as Linus recently stated (see the link below). That makes
the explanation also easier to find with a text search. For the same
reason and to improve comprehensibility provide an example, too.

Slightly improve the text at the same time to make it more obvious
developers are meant to add links to issue reports in mailing list
archives, as those allow regression tracking efforts to automatically
check which bugs got resolved.

Move the section also downwards slightly, to reduce jumping back and
forth between aspects relevant for the top and the bottom part of the
commit msg.

Link: https://lore.kernel.org/lkml/CAHk-=wgBhyLhQLPem1vybKNt7BKP+=qF=veBgc7VirZaXn4FUw@mail.gmail.com/


CC: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/27105768dc19b395e7c8e7a80d056d1ff9c570d0.1635152553.git.linux@leemhuis.info


[jc: tweaked wording following Konstantin's recommendation]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>