Documentation: Clarify Export, NoRd, internal usage

[bms63]

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Function Documentation

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All Functions should be well-documented and follow all our guidelines. However, some functions a user will never need in their code and we should reduce function-docs overload as much as possible.

We should clarify the use of @export, @noRd and @keyword internal and provide guidance on when to:

• export a function
• not export a function
• export a function but hide from reference page
• use of @nord -
• use of internal for keyword

See for example
pharmaverse/admiral#2220 (comment)