docs: bug-bisect: add a note about bisecting -next

Explicitly mention how to bisect -next, as nothing in the kernel tree
currently explains that bisects between -next versions won't work well
and it's better to bisect between mainline and -next.

Co-developed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/ec19d5fc503ff7db3d4c4ff9e97fff24cc78f72a.1730808651.git.linux@leemhuis.info