The documentation for grub-install in docs/grub.texi is mostly just a
leftover from the GRUB legacy documentation, and is therefore
inaccurate in many ways with respect to GRUB2. Attached is a merge
proposal that fixes most of the problems I found with the grub-install
documentation. One problem that I did find that I don't know quite how
to deal with (and haven't in this patch) is this paragraph:

By the way, as noted above, it is quite difficult to guess BIOS drives
correctly under a UNIX-like OS. Thus, grub-install will prompt you to
check if it could really guess the correct mappings, after the
installation. The format is defined in Device map. Please be quite
careful. If the output is wrong, it is unlikely that your computer
will be able to boot with no problem.

The device.map is not needed in most cases, and GRUB2 for all but a
few odd and uncommon cases is completely independent of device
enumeration.  Despite that, a user can create a device.map that would
cause problems with grub-install, for instance "(hd0) /dev/md0".  So
some mention of device.map with regard to grub-install is probably
needed, but I'll leave that to someone else to write.