Display "completions not supported" for resource templates which do not support completions

[kentcdodds]

Is your feature request related to a problem? Please describe.

When you have a resource template which does not support completions, it looks like this:

It's very possible you may not know how to support completions (😅).

Describe the solution you'd like

Display a message with a link to the spec about completions:

Describe alternatives you've considered

Status quo, people just "figure it out" eventually (like I did 😅).

Additional context

None.

[balajmarius]

I started working on this.