Doc update: Explain what drawing a line does in RectangularSelector.

Author: lkjell

lib/matplotlib/widgets.py

@@ -2026,6 +2026,7 @@ def __init__(self, ax, onselect, drawtype='box',
         a box or nothing between click and actual position by setting
 
         ``drawtype = 'line'``, ``drawtype='box'`` or ``drawtype = 'none'``.
+        Drawing a line would result in a line from vertex A to vertex C in a rectangle ABCD.
 
         *spancoords* is one of 'data' or 'pixels'.  If 'data', *minspanx*
         and *minspanx* will be interpreted in the same coordinates as