chore(render.display): Improve error message

[gadenbuie]

Changes the error thrown by @render.display when the decorated function has a return value from

@render.display functions should not return values. (`None` is allowed).

to

@render.display functions should not return values. Instead, the function
body should include Shiny Express code that will be dynamically rendered
in the app. (`None` is a valid return value.)

Notes and Questions

Is @render.display intended only for Shiny Express? (If yes, this does speak to having shiny.express.render and for not including render.display in shiny.)

@schloerke described @render.display as follows on Slack:

• render.ui renders the last thing returned.
• render.display renders every printable line within the function.

If render.display is intended for Shiny Core as well then we could fall back to "Instead, @render.display dynamically renders every printable line within the function body" rather than naming Shiny Express. This phrasing is technically more precise but still not quite actionable for someone just starting with Shiny or Express.

[schloerke]

Thank you for the suggestion within the PR!

I believe it is possible that render.display is used within Core. So I altered the phrasing as you suggested.