Test document selector clarification

[jrieken]

Refs: #21886

• any os @mjbvz
• any os @sandy081

Complexity: 2

When registering feature like IntelliSense et al, some extensions use language identifiers, like typescript, as document selector. That's fine and OK but also means that they receive documents that are not stored on disk. Often that comes to as a surprise to extensions authors and we want to add some clarification. Register a language-feature as described about and test that

• you get an information message (on the console) with a fwd-link to a help-page explaining the challenge
• you get that message only once per session
• you get that message only for the extension your are currently developing