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.  The wording I'm bringing in here matches what
873b9b6 (Add some text about extensions, 2016-06-26, opencontainers#164) landed for
the org.opencontainers.* annotation namespace.

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

descriptor.md

@@ -38,7 +38,7 @@ The following fields contain the primary properties that constitute a Descriptor
 
 ### Reserved
 
-The following field keys MUST NOT be used in descriptors specified in other OCI specifications:
+The following field keys are reserved and MUST NOT be used by other specifications.
 
 - **`data`** *string*