Tidy up documentation associated with window groups.

Tidy up documentation associated with window groups.

* doc/lispref/windows.texi (Basic Windows): Add an @anchor for "Window
Groups".  Correct example function to `window-group-start'.
(Window Start and End, Textual scrolling): Point to the new anchor.  State
that (most of) the args in window group functions have the same meaning as for
the corresponding window primitives.

* doc/lispref/positions.texi (Screen Lines).  Same as above.