descriptor: Rephrase reserved wording
descriptor: Rephrase reserved wording

We're not forbidding other OCI specs from using these fields, we're
forbidding all specs (except for future versions of image-spec) from
using these fields [1,2].  The wording I'm bringing in here matches
what 873b9b64 (Add some text about extensions, 2016-06-26, #164)
landed for the org.opencontainers.* annotation namespace.

My personal preference would be to use JSON-LD to assign explicit
semantics to every field (after which you don't have to worry about
namespacing the fields themselves) [3], but that would be a larger
change ;).

[1]: https://github.com/opencontainers/image-spec/pull/111#r65460193
[2]: https://github.com/opencontainers/image-spec/pull/111#r65659259
[3]: https://github.com/opencontainers/image-spec/pull/111#r65468737
     And later comments in that sub-thread.

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

jonboulle/image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,vbatts/oci-image-spec,opencontainers/image-spec,vbatts/oci-image-spec,opencontainers/image-spec,mattspencer-arm/image-spec