boot/grub2: Add some more explanation in ostree-grub-generator
boot/grub2: Add some more explanation in ostree-grub-generator

Even with the previous docstring, I didn't understand at first the
relationship between the `ostree-grub-generator` script and
`ostree-bootloader-grub2.c`. Throw some more docs to clarify things a
bit.

Closes: #1791
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree