* docs/grub.texi (Simple configuration): Document GRUB_CMDLINE_XEN

* docs/grub.texi (Simple configuration): Document GRUB_CMDLINE_XEN
and GRUB_CMDLINE_XEN_DEFAULT.  Recommend setting
GRUB_GFXPAYLOAD_LINUX=text rather than unsetting it in order to
disable gfxpayload.
(Shell-like scripting): Add real content.
(Serial terminal): Suggest `terminal_input serial; terminal_output
serial' rather than putting the two commands on separate lines,
since console input will be inoperative after the first command.
(menuentry): Document --class, --users, and --hotkey options.
(terminfo): Describe what `visually-ordered UTF-8' means (thanks,
Vladimir Serbinenko).