docs: verify/bisect: explain testing reverts, patches and newer code

Rename 'Supplementary tasks' to 'Complementary tasks' while introducing
a section 'Optional tasks: test reverts, patches, or later versions':
the latter is something readers occasionally will have to do after
reporting a bug and thus is best covered here.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/dacf26a4c48e9e8f04ecbc77e0a74c9b2a6a1103.1712647788.git.linux@leemhuis.info