reword documentation on nix-path config option

also make the fallback values explicit.

this is to clarify behavior. using the positive boolean is easier to
read, as it does not require flipping bits in your head.

src/libexpr/eval.hh

@@ -581,7 +581,31 @@ struct EvalSettings : Config
 
     Setting<Strings> nixPath{
         this, getDefaultNixPath(), "nix-path",
-        "List of directories to be searched for `<...>` file references."};
+        R"(
+          List of directories to be searched for file references in `<...>` search path expressions.
+
+          - The configuration setting is overridden by the [`NIX_PATH`](@docroot@/command-ref/env-common.md#env-NIX_PATH)
+          environment variable.
+          - `NIX_PATH` be extended using the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I).
+          - `NIX_PATH` is overridden by specifying the setting as the option `--nix-path`.
+          - Any current value is extended with `--extra-nix-path`.
+
+          If nothing is specified:
+
+          - If [restricted evaluation](#conf-restrict-eval) is enabled, use the values in `NIX_PATH`.
+
+          - Otherwise, and if `NIX_PATH` is not set, use a fallback:
+
+            ```
+            $HOME/.nix-defexpr/channels"
+            nixpkgs=$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.
+
+          If [pure evaluation](#conf-pure-eval) is enabled, search path expressions are not resolved at all.
+        )"};
 
     Setting<bool> restrictEval{
         this, false, "restrict-eval",
@@ -593,7 +617,10 @@ struct EvalSettings : Config
         )"};
 
     Setting<bool> pureEval{this, false, "pure-eval",
-        "Whether to restrict file system and network access to files specified by cryptographic hash."};
+        R"(
+          If set to `true`, restrict file system and network access to files specified by cryptographic hash,
+          and do not resolve `<...>` search path expressions.
+        )"};
 
     Setting<bool> enableImportFromDerivation{
         this, true, "allow-import-from-derivation",