Document API change in ContourSet

Also, update documentation for Attributes.

Followup to matplotlib#19623; fixes matplotlib#20906.

Author: QuLogic

doc/api/prev_api_changes/api_changes_3.5.0/behaviour.rst

@@ -184,6 +184,17 @@ Usually norms have a scale, and the advantage of having a  `~.scale.ScaleBase`
 attached to a norm is to provide a scale, and associated tick locators and
 formatters, for the colorbar.
 
+``ContourSet`` always use ``PathCollection``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to correct rendering issues with closed loops, the `.ContourSet` now
+creates a `.PathCollection` instead of a `.LineCollection` for line contours.
+This type matches the artist used for filled contours.
+
+This affects `.ContourSet` itself and its subclasses, `.QuadContourSet`
+(returned by `.Axes.contour`), and `.TriContourSet` (returned by
+`.Axes.tricontour`).
+
 ``hatch.SmallFilledCircles`` inherits from ``hatch.Circles``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

lib/matplotlib/contour.py

@@ -662,10 +662,9 @@ def _find_closest_point_on_path(xys, p):
 ax : `~matplotlib.axes.Axes`
     The Axes object in which the contours are drawn.
 
-collections : `.silent_list` of `.LineCollection`\s or `.PathCollection`\s
+collections : `.silent_list` of `.PathCollection`\s
     The `.Artist`\s representing the contour. This is a list of
-    `.LineCollection`\s for line contours and a list of `.PathCollection`\s
-    for filled contours.
+    `.PathCollection`\s for both line and filled contours.
 
 levels : array
     The values of the contour levels.