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
 
@@ -39,28 +39,35 @@ 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 realised in the Nix store.
 
 The following types of installable are supported by most commands:
 
+)"" + (!experimentalFeatureSettings.isEnabled(Xp::Flakes) ? "" : (std::string) R""(
 - [Flake output attribute](#flake-output-attribute)
+)"") + R""(
 - [Store path](#store-path)
 - [Nix file](#nix-file), optionally qualified by an attribute path
 - [Nix expression](#nix-expression), optionally qualified by an attribute path
 
 For most commands, if no installable is specified, `.` as assumed.
 That is, Nix will operate on the default flake output attribute of the flake in the current directory.
 
+)"" + (!experimentalFeatureSettings.isEnabled(Xp::Flakes) ? "" : (std::string) R""(
 ### Flake output attribute
 
 Example: `nixpkgs#hello`
@@ -119,6 +126,8 @@ subcommands, these are `packages.`*system*,
 attributes `packages.x86_64-linux.hello`,
 `legacyPackages.x86_64-linux.hello` and `hello`.
 
+)"") + R""(
+
 ### Store path
 
 Example: `/nix/store/v5sv61sszx301i0x6xysaqzla09nksnd-hello-2.10`
@@ -218,6 +227,8 @@ 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*. These are documented