Improve documentation for APP_* providers

[adam-kiss-sg]

I'm submitting a...

• Regression
• Bug report
• Feature request
• Documentation issue or request (new chapter/page)
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Documentation Request

APP_* providers (eg: a provider for APP_GUARD) work very differently from other custom providers. For example, you can register multiple providers for them, eg:

    {
      provide: APP_GUARD,
      useClass: AuthGuard('jwt'),
    },
    {
      provide: APP_GUARD,
      useClass: GroupGuard,
    }

They also are not "real" providers, in the sense that they can't be retreived from the DI container, eg this will return an error:

app.get(APP_GUARD)

For me it was kind of confusing, until I read the source code:
https://github.com/nestjs/nest/blob/master/packages/core/scanner.ts#L491

It might be worth to update the custom providers documentation with some more details about these.