docs(techniques): Make the use of ObjectId less ambiguous

[getTobiasNielsen]

Clarify how to get better typesafety when using mongoose.Schema.Types.ObjectId as a type in @Prop()

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• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

I want clear up some documentation that I think is confusing to people that are new to typescript / nestjs / mongoose.

I think it was not clear enough what the correct type should be if people choose to not populate the references on mongoose schemas.

I added some examples and comments about how you might go about adding mongoose.Schema.Types.ObjectIds to the schemas if you don't plan on always populating those.

Thank you for a lovely library.

Does this PR introduce a breaking change?

• Yes
• No

Other information