Add whatsnew note on F2 to open editor

I forgot to do this when I added the feature.

docs/source/whatsnew/pr/f2-edit-input.rst

@@ -0,0 +1,3 b''
+- You can now press F2 while typing at a terminal prompt to edit the contents
+  in your favourite terminal editor. Set the :envvar:`EDITOR` environment
+  variable to pick which editor is used.