I haven't located the section of the Help file that says that, but editing of subtitle text is done in the workspace (the part where menus and videos are displayed while you're editing).

Assuming you've followed the instructions to add a subtitle track and to add subtitle events to that track (or import subtitles from elsewhere), then select the subtitle event on the timeline. This will display that event's text box in the workspace. Select that text box and either press F2 or right-click the text box and choose Edit Text.

Rob