Clarify FigureBase.tight_bbox as different from all other artists.

It explicitly scales the union of child tightbboxes by 1/dpi, i.e.
converts from pixels to inches.  Perhaps that's not the most elegant
design, but let's make that behavior clear.

Author: anntzer

lib/matplotlib/figure.py

@@ -1596,7 +1596,10 @@ def get_default_bbox_extra_artists(self):
 
     def get_tightbbox(self, renderer, bbox_extra_artists=None):
         """
-        Return a (tight) bounding box of the figure in inches.
+        Return a (tight) bounding box of the figure *in inches*.
+
+        Note that `.FigureBase` differs from all other artists, which return
+        their `.Bbox` in pixels.
 
         Artists that have ``artist.set_in_layout(False)`` are not included
         in the bbox.