docs: update the guidance for Link: tags

As stated definitively by Linus, the use of Link: tags should be limited to
situations where there is additional useful information to be found at the
far end of the link.  Update our documentation to reflect that policy, and
to remove the suggestion for a Git hook to add those tags automatically.

Link: https://lore.kernel.org/all/CAHk-=wh5AyuvEhNY9a57v-vwyr7EkPVRUKMPwj92yF_K0dJHVg@mail.gmail.com/


Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <87segwyc3p.fsf@trenco.lwn.net>