rpk/registry: document print flag exclusivity in help text

The print-schema and print-metadata flags are marked mutually
exclusive via MarkFlagsMutuallyExclusive, but the help text does not
say so. State it in both flag descriptions and the long help, so users
(and the generated CLI reference docs, which are built from this help
text) see the constraint without hitting the runtime error.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>