docs/xl: Clarify documentation for mem-max and mem-set

docs/xl: Clarify documentation for mem-max and mem-set

mem-set is the primary command that users will need to use and
understand.  Move it first, and clarify the wording; also specify that
you can't set the target higher than maxmem from the domain config.

mem-max is actually a pretty useless command at the moment.  Clarify
that users are not expected to use it; and document all of its quirky
behavior.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Lars Kurth <lars.kurth@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>