Clarify the special role of default.nix

[fricklerhandwerk]

Problem

The fact that default.nix has a bit of a special treatment is scattered in various places, and there's evidence that new users struggle with that. There's no central place where that is explained unambiguously and all instances of special treatment listed in an overview.

Related: NixOS/nix.dev#198

Proposal

Add a page to the manual. Not sure where, as this is a cross-cutting concern shared by the language evaluator and commands that take paths to Nix files.

Here's a good summary: https://stackoverflow.com/questions/44088192/when-and-how-should-default-nix-shell-nix-and-release-nix-be-used

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

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