docs(nix): mention +lsp flag

Ref: #6592

modules/lang/nix/README.org

@@ -22,6 +22,12 @@ Includes:
 - +tree-sitter ::
   Leverages tree-sitter for better syntax highlighting and structural text
   editing. Requires [[doom-module::tools tree-sitter]].
+- +lsp ::
+  Enable an LSP hook for ~nix-mode~. Requires [[doom-module::tools lsp]] and a language
+  server (one of either ~nil~ or ~rnix-lsp~).
+
+  You can still start a nix lsp manually without this flag, this just adds
+  a hook to always start the lsp when loading ~nix-mode~.
 
 ** Packages
 - [[doom-package:company-nixos-options]] if [[doom-module::completion company]]