docs/process/xen-release-management: Lesson to learn

docs/process/xen-release-management: Lesson to learn

The 4.10 release preparation was significantly more hairy than ideal.
(We seem to have a good overall outcome despite, rather than because
of, our approach.)

This is the second time (at least) that we have come close to failure
by committing to a release date before the exact code to be released
is known and has been made and tested.

Evidently our docs makes it insufficiently clear not to do that.

CC: Julien Grall <julien.grall@arm.com>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Lars Kurth <lars.kurth@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>