Add tmux_attach_start script zsh completions

Will autocomplete with currently open tmux sessions.

tmux/.local/bin/tmux_attach_start

@@ -120,6 +120,7 @@ _attach_or_create() {
   if ! _session_exists; then
     _create_detached_session
     # look for .tmux.session file if no file has been explicitly passed in
+    # session files always take precedence over environment files when specifically passed in
     if ! _file_exists "$session_file"; then
       _load_env_session_file
     fi