Documentation: best practices for using Link trailers

Based on multiple conversations, most recently on the ksummit mailing
list [1], add some best practices for using the Link trailer, such as:

- how to use markdown-like bracketed numbers in the commit message to
indicate the corresponding link
- when to use lore.kernel.org vs patch.msgid.link domains

Cc: [email protected]
Link: https://lore.kernel.org/20240617-arboreal-industrious-hedgehog-5b84ae@meerkat # [1]
Signed-off-by: Konstantin Ryabitsev <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/20240619-docs-patch-msgid-link-v2-2-72dd272bfe37@linuxfoundation.org

Author: mricon

Documentation/process/maintainer-tip.rst

@@ -372,17 +372,31 @@ following tag ordering scheme:
 
  - Link: ``https://link/to/information``
 
-   For referring to an email on LKML or other kernel mailing lists,
-   please use the lore.kernel.org redirector URL::
+   For referring to an email posted to the kernel mailing lists, please
+   use the lore.kernel.org redirector URL::
 
-     https://lore.kernel.org/r/email-message@id
+     Link: https://lore.kernel.org/email-message-id@here
 
-   The kernel.org redirector is considered a stable URL, unlike other email
-   archives.
+   This URL should be used when referring to relevant mailing list
+   topics, related patch sets, or other notable discussion threads.
+   A convenient way to associate ``Link:`` trailers with the commit
+   message is to use markdown-like bracketed notation, for example::
 
-   Maintainers will add a Link tag referencing the email of the patch
-   submission when they apply a patch to the tip tree. This tag is useful
-   for later reference and is also used for commit notifications.
+     A similar approach was attempted before as part of a different
+     effort [1], but the initial implementation caused too many
+     regressions [2], so it was backed out and reimplemented.
+
+     Link: https://lore.kernel.org/some-msgid@here # [1]
+     Link: https://bugzilla.example.org/bug/12345  # [2]
+
+   You can also use ``Link:`` trailers to indicate the origin of the
+   patch when applying it to your git tree. In that case, please use the
+   dedicated ``patch.msgid.link`` domain instead of ``lore.kernel.org``.
+   This practice makes it possible for automated tooling to identify
+   which link to use to retrieve the original patch submission. For
+   example::
+
+     Link: https://patch.msgid.link/patch-source-message-id@here
 
 Please do not use combined tags, e.g. ``Reported-and-tested-by``, as
 they just complicate automated extraction of tags.