Start cross-referencing experimental features

- Create a glossary entry for experimental features.

- Have the man page experimental feature notice link `nix-commmand`.

  (Eventually this should be programmed, based on whether the command is
  experimental, and if so what experimental feature does it depend on.)

- Document which installables depend on which experimental features.

  I tried to use the same style (bold warning and block quote) that the
  top of the man page uses.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/nix/nix.md

@@ -48,12 +48,17 @@ manual](https://nixos.org/manual/nix/stable/).
 
 # Installables
 
+> **Warning** \
+> Installables are part of the unstable
+> [`nix-command` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-nix-command),
+> and subject to change without notice.
+
 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:
 
-- [Flake output attribute](#flake-output-attribute)
+- [Flake output attribute](#flake-output-attribute) (experimental)
 - [Store path](#store-path)
 - [Nix file](#nix-file), optionally qualified by an attribute path
 - [Nix expression](#nix-expression), optionally qualified by an attribute path
@@ -63,6 +68,13 @@ That is, Nix will operate on the default flake output attribute of the flake in
 
 ### Flake output attribute
 
+> **Warning** \
+> Flake output attribute installables depend on both the
+> [`flakes`](@docroot@/contributing/experimental-features.md#xp-feature-flakes)
+> and
+> [`nix-command`](@docroot@/contributing/experimental-features.md#xp-feature-nix-command)
+> experimental features, and subject to change without notice.
+
 Example: `nixpkgs#hello`
 
 These have the form *flakeref*[`#`*attrpath*], where *flakeref* is a