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]: opencontainers/image-spec#111 (comment)
[2]: opencontainers/image-spec#111 (comment)
[3]: opencontainers/image-spec#111 (comment)
     And later comments in that sub-thread.

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

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*