Gate nix --help sections based on stability

This frequent unescaping might not be so nice looking, but we can do it
a different way. The behavior I think is good.

src/nix/nix.md

@@ -1,4 +1,4 @@
-R""(
+(!experimentalFeatureSettings.isEnabled(Xp::Flakes) ? "" : (std::string) R""(
 
 # Examples
 
@@ -39,19 +39,25 @@ R""(
   Hello, world!
   ```
 
+)"") + R""(
+
 # Description
 
 Nix is a tool for building software, configurations and other
 artifacts in a reproducible and declarative way. For more information,
 see the [Nix homepage](https://nixos.org/) or the [Nix
 manual](https://nixos.org/manual/nix/stable/).
 
+)"" + (!experimentalFeatureSettings.isEnabled(Xp::NixCommand) ? "" : ((std::string) R""(
+
 # Installables
 
 Many `nix` subcommands operate on one or more *installables*. These are
 command line arguments that represent something that can be built in
 the Nix store. Here are the recognised types of installables:
 
+)"" + (!experimentalFeatureSettings.isEnabled(Xp::Flakes) ? "" : (std::string) R""(
+
 * **Flake output attributes**: `nixpkgs#hello`
 
   These have the form *flakeref*[`#`*attrpath*], where *flakeref* is a
@@ -108,6 +114,8 @@ the Nix store. Here are the recognised types of installables:
   attributes `packages.x86_64-linux.hello`,
   `legacyPackages.x86_64-linux.hello` and `hello`.
 
+)"") + R""(
+
 * **Store paths**: `/nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10`
 
   These are paths inside the Nix store, or symlinks that resolve to a
@@ -208,6 +216,8 @@ operate are determined as follows:
 
 * Otherwise, Nix will use all outputs of the derivation.
 
+)"")) + R""(
+
 # Nix stores
 
 Most `nix` subcommands operate on a *Nix store*.