Add note to UPDATING for ino64 to follow the standard upgrade process

The existing upgrade process documented in UPDATING is both necessary
and sufficient for upgrading across the ino64 change. However, the
shortcut of installing both kernel + world before a single reboot has
been possible for quite some time, and several developers and users
were surprised by fallout from ino64. Add an explicit entry pointing
out that the full process must be followed.

Reviewed by: allanjude, gjb, vangyzen
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10877