doc: clarify Backport-PR-URL metadata added automatically

Signed-off-by: M McCready <66998419+MikeMcC399@users.noreply.github.com>
PR-URL: #62668
Fixes: #61955
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: MikeMcC399

doc/contributing/backporting-to-release-lines.md

@@ -75,8 +75,8 @@ line.
 5. Resolve conflicts using `git add` and `git cherry-pick --continue`.
 
 6. Leave the commit message as is. If you think it should be modified, comment
-   in the pull request. The `Backport-PR-URL` metadata does need to be added to
-   the commit, but this will be done later.
+   in the pull request. Do not manually add a `Backport-PR-URL` metadata entry.
+   This is automatically added later.
 
 7. Verify that `make -j4 test` passes.