config: Replace "SHOULD consider" for 'rootfs' path value
config: Replace "SHOULD consider" for 'rootfs' path value

SHOULD means that [1]:

  ... there may exist valid reasons in particular circumstances to
  ignore a particular item, but the full implications must be
  understood and carefully weighed before choosing a different course.

So consideration is already baked into the definition, and we can just
say "SHOULD be" and warn folks whenever they use a value other than
'rootfs'.

Also move this under the non-Windows conditions, because the advice
they doesn't apply to Windows where the value MUST be a volume GUID
path (since 2283e63, Windows: Remove Sandbox, additional tweaks,
2017-05-23, #849).

[1]: https://tools.ietf.org/html/rfc2119#section-3

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

wking/opencontainer-runtime-spec,opencontainers/specs,laijs/specs,wking/opencontainer-specs,hqhq/runtime-spec,opencontainers/runtime-spec,hqhq/specs