doc: /* Return position information for buffer position POS in WINDOW.
POS defaults to point in WINDOW; WINDOW defaults to the selected window.
+If POS is in invisible text or is hidden by `display' properties,
+this function may report on buffer positions before or after POS.
+
Return nil if POS is not visible in WINDOW. Otherwise,
the return value is similar to that returned by `event-start' for
a mouse click at the upper left corner of the glyph corresponding