reword documentation on nix-path config option (#7772)

* docs: unify documentation on search paths

- put all the information on search path semantics into `builtins.findFile`
- put all the information on determining the value of `builtins.nixPath` into the
  `nix-path` setting

  maybe `builtins.nixPath` is a better place for this, but those bits
  can still be moved around now that it's all next to each other.
- link to the syntax page for lookup paths from all places that are
  concerned with it
- add or clarify examples
- add a test verifying a claim from documentation

src/libexpr/eval-settings.hh

@@ -79,19 +79,24 @@ struct EvalSettings : Config
           This setting determines the value of
           [`builtins.nixPath`](@docroot@/language/builtins.md#builtins-nixPath) and can be used with [`builtins.findFile`](@docroot@/language/builtins.md#builtins-findFile).
 
-          The default value is
+          - The configuration setting is overridden by the [`NIX_PATH`](@docroot@/command-ref/env-common.md#env-NIX_PATH)
+          environment variable.
+          - `NIX_PATH` is overridden by [specifying the setting as the command line flag](@docroot@/command-ref/conf-file.md#command-line-flags) `--nix-path`.
+          - Any current value is extended by the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) or `--extra-nix-path`.
 
-          ```
-          $HOME/.nix-defexpr/channels
-          nixpkgs=$NIX_STATE_DIR/profiles/per-user/root/channels/nixpkgs
-          $NIX_STATE_DIR/profiles/per-user/root/channels
-          ```
+          If the respective paths are accessible, the default values are:
 
-          It can be overridden with the [`NIX_PATH` environment variable](@docroot@/command-ref/env-common.md#env-NIX_PATH) or the [`-I` command line option](@docroot@/command-ref/opt-common.md#opt-I).
+          - `$HOME/.nix-defexpr/channels`
+          - `nixpkgs=$NIX_STATE_DIR/profiles/per-user/root/channels/nixpkgs`
+          - `$NIX_STATE_DIR/profiles/per-user/root/channels`
+
+          See [`NIX_STATE_DIR`](@docroot@/command-ref/env-common.md#env-NIX_STATE_DIR) for details.
 
           > **Note**
           >
-          > If [pure evaluation](#conf-pure-eval) is enabled, `nixPath` evaluates to the empty list `[ ]`.
+          > If [restricted evaluation](@docroot@/command-ref/conf-file.md#conf-restrict-eval) is enabled, the default value is empty.
+          >
+          > If [pure evaluation](#conf-pure-eval) is enabled, `builtins.nixPath` *always* evaluates to the empty list `[ ]`.
         )", {}, false};
 
     Setting<std::string> currentSystem{