annotations: Use SPDX License Expressions for licenses

Instead of comma-separated short identifiers, which have unclear
semantics (are the delimiters AND or OR?).  I don't see any discussion
of the syntax for this field in [1] (which landed it), but I'd floaded
license expressions before in the sub-thread starting at [2].  Greg
had pushed back against my earlier proposal (licensing information on
descriptors) with [3]:

> No, that's not going to work at all, you can't properly describe the
> license for a whole layer in any form of a string that could be
> standardized or parsed. SPDX is great for describing the individual
> licenses of things, but not for a collection of things, which almost
> always has an arbitrary license (example, what's the license, in a
> simple string, for a Ubuntu base layer?)

But SPDX License Expression are both more expressive and better
defined than the current comma delimiters.  Everything you could have
said with the comma-delimited string you can say more clearly with a
SPDX License Expression.  And because the syntax is not OCI-specific,
you're more likely to be able to find tooling that handles these
values out of the box.

[1]: #636
[2]: #501 (comment)
[3]: #501 (comment)

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

annotations.md

@@ -25,7 +25,7 @@ This specification defines the following annotation keys, intended for but not l
 * **org.opencontainers.image.version** [Semantic versioning-compatible](http://semver.org/) version of the packaged software. The version MAY match a label or tag in the source code repository.
 * **org.opencontainers.image.revision** Source control revision identifier for the packaged software.
 * **org.opencontainers.image.vendor** Name of the distributing entity, organization or individual.
-* **org.opencontainers.image.licenses** Comma-separated list of licenses under which contained software is distributed, in [SPDX Short identifier](https://spdx.org/licenses/) form.
+* **org.opencontainers.image.licenses** License(s) under which contained software is distributed as a [SPDX License Expression][spdx-license-expression].
 * **org.opencontainers.image.ref.name** Name of the reference for a target (string). SHOULD only be considered valid when on descriptors on `index.json` within [image layout](image-layout.md).
 * **org.opencontainers.image.name** Human-readable name of the software packaged in the image (string)
 * **org.opencontainers.image.description** Human-readable description of the software packaged in the image (string)
@@ -52,3 +52,5 @@ While users are encouraged to use the **org.opencontainers.image** keys, tools M
 | `ref.name` | | No equivalent in Label Schema |
 | | `schema-version`| No equivalent in the OCI Image Spec |
 | | `docker.*`, `rkt.*` | No equivalent in the OCI Image Spec |
+
+[spdx-license-expression]: https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60