d/[..]/grub.d/xen.cfg: command line "starter kit"

As pointed out by Gergely in debian bug #919758, the examples in the
grub documentation contain incorrect suggestions, at least for dom0_mem
and earlyprintk.

Correct those, and take the opportunity to refresh all of this a bit,
including the most common set of used options.

Also, point to the online documentation where more explanation about all
options can be found.

(Closes: #919758)