docs: verify/bisect: improve install instructions

These changes among others ensure modules will be installed when
/sbin/installkernel is missing. Furthermore describe better what tasks
the script ideally performs so that users can more easily check if those
have been taken care of. In addition to that point to the distro's
documentation for further details on installing kernels manually.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <e392bd5eb12654bed635f32b24304a712b0c67d1.1710750972.git.linux@leemhuis.info>