doc: contrib guidelines: modify rule about adding links

Clarify linking issues in PRs and Commit messages and address the issue
of ambiguity across multiple github repos.

Not allowing links in commit messages so that forks can be happy is not
great.

Forks on Github have no problem with linking, however,
disconnected forks seem to not link correctly, but then why does this
matter?

If someone is taking Zephyr and developing it as their own, they
can for sure do that, but why this should force the Zephyr project to
manage traceability differently and make it more difficult for
developers to add such information in the git metadata directly and not
relying on pull requests.

Most developers use git directly and rely less on pull request and
github UI when browsing changes in the code base.. We need to be able to
see by looking at git commits and git
history if a commit is associated with a bug, some PRs might fix
multiple issues, so the context is important and by looking at each
commit the trace can be established immesiately without going back to
github pull requests.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

doc/contribute/guidelines.rst

@@ -481,31 +481,41 @@ in the Git commit's ``Author:`` field.
 See the :ref:`contributor-expectations` for a more complete discussion of
 contributor and reviewer expectations.
 
-Adding links
+Adding Links
 ------------
 
 .. _GitHub references:
    https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/autolinked-references-and-urls
 
-Do not include `GitHub references`_ in the commit message directly, as it can
-lose meaning in case the repository is forked, for example. Instead, if the
-change addresses a specific GitHub issue, include in the Pull Request message a
-line of the form:
+If your change addresses a specific GitHub issue, include a reference in the
+pull request description using the following format:
+
+.. code-block:: none
+
+   Fixes zephyrproject-rtos/zephyr#[issue number]
+
+For pull requests to the Zephyr project only, the short form can also be used,
+for example:
 
 .. code-block:: none
 
    Fixes #[issue number]
 
-Where ``[issue number]`` is the relevant GitHub issue's number. For
-example:
+Replace [issue number] with the relevant GitHub issue number. For example:
 
 .. code-block:: none
 
-   Fixes: #1234
+   Fixes zephyrproject-rtos/zephyr#1234
+
+This syntax ensures that the issue is automatically closed when the pull
+request is merged. Always specify the full repository path
+(zephyrproject-rtos/zephyr) to avoid ambiguity, especially when working across
+multiple repositories.
+
+The same format can also be used in commit messages.
 
-You can point to other relevant information that can be found on the web using
-:code:`Link:` tags. This includes, for example: GitHub issues, datasheets,
-reference manuals, etc.
+For linking to additional external resources—such as related issues,
+datasheets, or technical reference manuals—use the ``Link:`` tag:
 
 .. code-block:: none