docs: verify/bisect: check taint flag

Instruct readers to check the taint flag, as the reason why it's set
might directly or indirectly cause the bug or interfere with testing.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <8fcaffa8e85f36d51178d61061355c3c8bc85a0f.1710750972.git.linux@leemhuis.info>