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>

doc/manual/generate-manpage.nix

@@ -10,7 +10,9 @@ let
 
       result = ''
         > **Warning** \
-        > This program is **experimental** and its interface is subject to change.
+        > This program is
+        > [**experimental**](@docroot@/contributing/experimental-features.md#xp-feature-nix-command)
+        > and its interface is subject to change.
 
         # Name
 

doc/manual/src/command-ref/experimental-commands.md

@@ -1,6 +1,6 @@
 # Experimental Commands
 
-This section lists experimental commands.
+This section lists [experimental commands](@docroot@/contributing/experimental-features.md#xp-feature-nix-command).
 
 > **Warning**
 >

doc/manual/src/glossary.md

@@ -225,3 +225,9 @@
     [string]: ./language/values.md#type-string
     [path]: ./language/values.md#type-path
     [attribute name]: ./language/values.md#attribute-set
+
+  - [experimental feature]{#gloss-experimental-feature}\
+    Not yet stabilized functionality guarded by named experimental feature flags.
+    These flags are enabled or disabled with the [`experimental-features`](./command-ref/conf-file.html#conf-experimental-features) setting.
+
+    See the contribution guide on the [purpose and lifecycle of experimental feaures](@docroot@/contributing/experimental-features.md).

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

@@ -208,12 +208,26 @@ Derivations can declare some infrequently used optional attributes.
     about converting to and from base-32 notation.)
 
   - [`__contentAddressed`]{#adv-attr-__contentAddressed}
-    If this **experimental** attribute is set to true, then the derivation
+    > **Warning**
+    > This attribute is part of an [experimental feature](@docroot@/contributing/experimental-features.md).
+    >
+    > To use this attribute, you must enable the
+    > [`ca-derivations`](@docroot@/contributing/experimental-features.md#xp-feature-ca-derivations) experimental feature.
+    > For example, in [nix.conf](../command-ref/conf-file.md) you could add:
+    >
+    > ```
+    > extra-experimental-features = ca-derivations
+    > ```
+
+    If this attribute is set to `true`, then the derivation
     outputs will be stored in a content-addressed location rather than the
     traditional input-addressed one.
-    This only has an effect if the `ca-derivations` experimental feature is enabled.
 
-    Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above).
+    Setting this attribute also requires setting
+    [`outputHashMode`](#adv-attr-outputHashMode)
+    and
+    [`outputHashAlgo`](#adv-attr-outputHashAlgo)
+    like for *fixed-output derivations* (see above).
 
   - [`passAsFile`]{#adv-attr-passAsFile}\
     A list of names of attributes that should be passed via files rather
@@ -307,9 +321,11 @@ Derivations can declare some infrequently used optional attributes.
 
   - [`unsafeDiscardReferences`]{#adv-attr-unsafeDiscardReferences}\
     > **Warning**
-    > This is an experimental feature.
+    > This attribute is part of an [experimental feature](@docroot@/contributing/experimental-features.md).
     >
-    > To enable it, add the following to [nix.conf](../command-ref/conf-file.md):
+    > To use this attribute, you must enable the
+    > [`discard-references`](@docroot@/contributing/experimental-features.md#xp-feature-discard-references) experimental feature.
+    > For example, in [nix.conf](../command-ref/conf-file.md) you could add:
     >
     > ```
     > extra-experimental-features = discard-references

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