Fix and document components of mouse event position.

Fix and document components of mouse event position.

 src/keyboard.c (make_lispy_position): Put a meaningful value in yret
 when the click is on the header or mode line.
 doc/lispref/commands.texi (Click Events): Document the values of X, Y and
 COL, ROW in the event's position, when the click is on the header or
 mode line, on the fringes, or in the margins.