doc,vm,test: remove _sandbox_ from vm documentation

The vm documentation uses the word _sandbox_ but has a significant
disclaimer about how that should not be interpreted to connote security
guarantees. Remove the term "sandbox" from documentation. As a result,
some code and tests also need to change.

PR-URL: #31057
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>