Move documentation for impure derivation

doc/manual/src/language/advanced-attributes.md

@@ -329,3 +329,40 @@ Derivations can declare some infrequently used optional attributes.
     This is useful, for example, when generating self-contained filesystem images with
     their own embedded Nix store: hashes found inside such an image refer
     to the embedded store and not to the host's Nix store.
+
+  - [`__impure`]{#adv-attr-__impure}\
+    > **Warning**
+	  > This is an experimental feature.
+    >
+    > To enable it, add the following to [nix.conf](../command-ref/conf-file.md):
+    >
+    > ```
+  	> extra-experimental-features = impure-derivations
+	  > ```
+
+    If the **experimental** attribute `__impure` is set to `true`, then the
+    derivation won't produce a fixed output. This means that an impure
+    derivation can have different outputs each time it is built.
+
+    Example:
+
+    ```
+    derivation {
+      name = "impure";
+      builder = /bin/sh;
+      __impure = true; # mark this derivation as impure
+      args = [ "-c" "read -n 10 random < /dev/random; echo $random > $out" ];
+      system = builtins.currentSystem;
+    }
+    ```
+
+    Each time this derivation is built it produces a different output,
+    since the builder outputs random bytes to $out.
+
+    > **Note**
+    > 
+    > Impure derivations have the following traits:
+    >   - They have access to the network
+    >   - Only fixed-output and other impure derivations can depend on
+    >     impure derivations
+    >   - They cannot be [content-addressed](@docroot@/contributing/experimental-features.md#xp-feature-ca-derivations)

src/libutil/experimental-features.cc

@@ -28,28 +28,8 @@ constexpr std::array<ExperimentalFeatureDetails, 11> xpFeatureDetails = {{
         .tag = Xp::ImpureDerivations,
         .name = "impure-derivations",
         .description = R"(
-            Allow derivations to produce non-fixed outputs by setting the
-            `__impure` derivation attribute to `true`. An impure derivation can
-            have differing outputs each time it is built.
-
-            Example:
-
-            ```
-            derivation {
-              name = "impure";
-              builder = /bin/sh;
-              __impure = true; # mark this derivation as impure
-              args = [ "-c" "read -n 10 random < /dev/random; echo $random > $out" ];
-              system = builtins.currentSystem;
-            }
-            ```
-
-            Each time this derivation is built, it can produce a different
-            output (as the builder outputs random bytes to `$out`).  Impure
-            derivations also have access to the network, and only fixed-output
-            or other impure derivations can rely on impure derivations. Finally,
-            an impure derivation cannot also be
-            [content-addressed](#xp-feature-ca-derivations).
+            Allow derivations to produce non-fixed outputs. See
+            [__impure](@docroot@/language/advanced-attributes.md#adv-attr-__impure) for details.
         )",
     },
     {