nixos/nh: change FLAKE variable to NH_FLAKE; improve option docs (#401255)

nixos/modules/programs/nh.nix

@@ -8,7 +8,10 @@ let
   cfg = config.programs.nh;
 in
 {
-  meta.maintainers = [ lib.maintainers.viperML ];
+  meta.maintainers = with lib.maintainers; [
+    NotAShelf
+    viperML
+  ];
 
   options.programs.nh = {
     enable = lib.mkEnableOption "nh, yet another Nix CLI helper";
@@ -19,9 +22,18 @@ in
       type = lib.types.nullOr lib.types.path;
       default = null;
       description = ''
-        The path that will be used for the `FLAKE` environment variable.
+        The path that will be used for the `NH_FLAKE` environment variable.
 
-        `FLAKE` is used by nh as the default flake for performing actions, like `nh os switch`.
+        `NH_FLAKE` is used by nh as the default flake for performing actions, such as
+        `nh os switch`. This behaviour can be overriden per-command with environment
+        variables that will take priority.
+
+        - `NH_OS_FLAKE`: will take priority for `nh os` commands.
+        - `NH_HOME_FLAKE`: will take priority for `nh home` commands.
+        - `NH_DARWIN_FLAKE`: will take priority for `nh darwin` commands.
+
+        The formerly valid `FLAKE` is now deprecated by nh, and will cause hard errors
+        in future releases if `NH_FLAKE` is not set.
       '';
     };
 
@@ -77,7 +89,7 @@ in
     environment = lib.mkIf cfg.enable {
       systemPackages = [ cfg.package ];
       variables = lib.mkIf (cfg.flake != null) {
-        FLAKE = cfg.flake;
+        NH_FLAKE = cfg.flake;
       };
     };