runtime: Drop "not supported by the base OS" loophole
runtime: Drop "not supported by the base OS" loophole

All of these operations should be supported on all of our
compliance-tested platforms.  If there are cases where a given OS
cannot support one of these operations, it should be discussed in that
operation's section, and the "Unless otherwise stated" qualifier makes
space for that.  But leaving the reader to decide on whether a host OS
supports a given operation seems too unstable for a specification.

I've also dropped the "OCI compliant" qualifier, because the (not)
compliant language in spec.md already ties OCI compliance to the RFC
2119 language.  There's no point in mentioning "OCI compliant" outside
of that section, because the "MUST" already brings in all of the
compliance associations we need.

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

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