Clarify mandoc

src/doc/man/cargo-metadata.adoc

@@ -161,7 +161,7 @@ The output has the following format:
                 }
             },
             /* List of registries to which this package may be published.
-               This is null if not specified. */
+               Publishing is unrestricted if null, and forbidden if an empty array. */
             "publish": [
                 "crates-io"
             ],