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>

4 files changed, 31 insertions, 7 deletions

diff --git a/doc/manual/generate-manpage.nix b/doc/manual/generate-manpage.nix
index 86f2ca567..0b3a23801 100644
--- a/doc/manual/generate-manpage.nix
+++ b/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
 
diff --git a/doc/manual/src/command-ref/experimental-commands.md b/doc/manual/src/command-ref/experimental-commands.md
index cfa6f8b73..286ddc6d6 100644
--- a/doc/manual/src/command-ref/experimental-commands.md
+++ b/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**
 >
diff --git a/doc/manual/src/glossary.md b/doc/manual/src/glossary.md
index 4eedb2e93..a9782be5c 100644
--- a/doc/manual/src/glossary.md
+++ b/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).
diff --git a/doc/manual/src/language/advanced-attributes.md b/doc/manual/src/language/advanced-attributes.md
index 3e8c48890..307971434 100644
--- a/doc/manual/src/language/advanced-attributes.md
+++ b/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