config: Clarify ociVersion covering the configuration <-> runtime API

There are other APIs described in this specification (e.g. the state
JSON format, and the in-flight command-line API [1]), but this string
covers the configuration file and referenced objects (e.g. the
filesystem at root.path).  As additional, backwards compatible
features are added to the spec (leading to 1.1, 1.2, etc. releases)
and supported by runtimes, those runtimes will *still* stupport 1.0
configs.  Once a 2.0 spec is cut, runtimes that only support 2.0 (and
nothing in the 1.0 line) will no longer support the 1.0 config.

My preferred approach here would be to use JSON-LD [2,3,4] to
explicitly document the intended semantics for each field, which would
allow us to drop the config-wide version and version each field
independently.  That would mean a breaking change on a particular
field would only break compatibility for folks who were using that
field.  Unfortunately, I haven't had much luck pushing the consensus
in that direction.

This commit does not add wording about how the runtime and other
consumers should handle an incompatible version.  We can address that
once the command-line API lands.

[1]: opencontainers#513
[2]: opencontainers#371 (comment)
[3]: opencontainers/image-spec#111 (comment)
[4]: opencontainers#510 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -12,7 +12,7 @@ Below is a detailed description of each field defined in the configuration forma
 
 * **`ociVersion`** (string, required) MUST be in [SemVer v2.0.0](http://semver.org/spec/v2.0.0.html) format and specifies the version of the OpenContainer specification with which the bundle complies.
 The OpenContainer spec follows semantic versioning and retains forward and backward compatibility within major versions.
-For example, if an implementation is compliant with version 1.0.1 of the spec, it is compatible with the complete 1.x series.
+For example, if a configuration is compliant with version 1.1 of this specification, it is compatible with all runtimes that support any 1.1 or later release of this specification, but it not compatible with a runtime that supports 1.0 and not 1.1.
 
 ### Example