diff --git a/doc/manual/src/contributing/documentation.md b/doc/manual/src/contributing/documentation.md
index 88b0bdaa9..e7f94ab8c 100644
--- a/doc/manual/src/contributing/documentation.md
+++ b/doc/manual/src/contributing/documentation.md
@@ -206,3 +206,22 @@ or inside `nix-shell` or `nix develop`:
 # make internal-api-html
 # xdg-open ./outputs/doc/share/doc/nix/internal-api/html/index.html
 ```
+
+## C API documentation (experimental)
+
+[C API documentation] is available online.
+You can also build and view it yourself:
+
+[C API documentation]: https://hydra.nixos.org/job/nix/master/external-api-docs/latest/download-by-type/doc/external-api-docs
+
+```console
+# nix build .#hydraJobs.external-api-docs
+# xdg-open ./result/share/doc/nix/external-api/html/index.html
+```
+
+or inside `nix-shell` or `nix develop`:
+
+```
+# make external-api-html
+# xdg-open ./outputs/doc/share/doc/nix/external-api/html/index.html
+```