Better document the semantics of get_text_width_height_descent.

Including whitespace seems to be the current behavior for agg and makes
right-alignment works as it does.  It is also necessary to make e.g. the
currently PR'ed rainbow_text (based on `annotation(..., xycoords=textobj)`
implementation work.

Documenting the behavior is in particular useful for third-party backend
implementers (mplcairo got this wrong previously).

Author: anntzer

lib/matplotlib/backend_bases.py

@@ -636,9 +636,10 @@ def _draw_text_as_path(self, gc, x, y, s, prop, angle, ismath):
 
     def get_text_width_height_descent(self, s, prop, ismath):
         """
-        Get the width, height, and descent (offset from the bottom
-        to the baseline), in display coords, of the string *s* with
-        `.FontProperties` *prop*.
+        Get the width, height, and descent (offset from the bottom to the baseline), in
+        display coords, of the string *s* with `.FontProperties` *prop*.
+
+        Whitespace at the start and the end of *s* is included in the reported width.
         """
         fontsize = prop.get_size_in_points()