docs: add self-attrs documentation (backport #13726)

src/nix/flake.md

@@ -573,6 +573,27 @@ The value of the `follows` attribute is a `/`-separated sequence of
 input names denoting the path of inputs to be followed from the root
 flake.
 
+## Self-attributes
+
+Flakes can declare attributes about themselves that affect how they are fetched.
+These attributes are specified using the special `self` input and are retroactively
+applied to it:
+
+```nix
+{
+  inputs.self.submodules = true;
+  inputs.self.lfs = true;
+}
+```
+
+The following self-attributes are supported:
+
+* `submodules`: A Boolean denoting whether Git submodules should be fetched when this flake is used as an input. When set to `true`, Git submodules will be automatically fetched without requiring callers to specify `submodules=1` in the flake reference URL. Defaults to `false`.
+
+* `lfs`: A Boolean denoting whether Git LFS (Large File Storage) files should be fetched when this flake is used as an input. When set to `true`, Git LFS files will be automatically fetched. Defaults to `false`.
+
+These self-attributes eliminate the need for consumers of your flake to manually specify fetching options in their flake references.
+
 Overrides and `follows` can be combined, e.g.
 
 ```nix