Describe a notion of root (or similar) for source tree abstraction paths

[roberth]

Problem

Currently Nix only knows one root, /., but that changes with lazy trees.

• Lazy trees #6530

Proposal

Document how paths work, and explain that multiple roots exists.

A root could be defined as any path that doesn't have a parent.

• Or has itself as parent if you want to define it that way (dirOf does...)
  Also note that a file may be a root
• Unless that possibility is removed. We might always create an unnecessary directory around such files.

TODO

• Have lazy trees
• Document path root, give it an anchor
• Refer here from nixpkgs path library docs.

Checklist

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

Priorities

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