docs(tree-sitter): document +tree-sitter flag in :lang readmes

Co-authored-by: Jeetaditya Chatterjee <jeetelongname@gmail.com>

modules/lang/elixir/README.org

@@ -16,6 +16,9 @@ This module provides support for [[https://elixir-lang.org/][Elixir programming
 - +lsp ::
   Enable LSP support for ~elixir-mode~. Requires [[doom-module:][:tools lsp]] and a langserver
   (supports [[https://github.com/elixir-lsp/elixir-ls/][elixir-ls]]).
+- +tree-sitter ::
+  Leverages tree-sitter for better syntax highlighting and structural text
+  editing. Requires [[doom-module:][:tools tree-sitter]].
 
 ** Packages
 - [[doom-package:][alchemist]]