The Artist property rasterized cannot be None anymore (#18989)

* The Artist property rasterized cannot be None anymore

It is now a bool only. Before the default was non and
`Artist.set_rasterized` documented to accept *None*. However, *None*
did not have a special meaning and was treated as *False.

* DOC: add missing markup

* Fix unresolved reference.

Co-authored-by: Thomas A Caswell <tcaswell@gmail.com>

doc/api/next_api_changes/behavior/18988-TH.rst

@@ -0,0 +1,5 @@
+The Artist property *rasterized* cannot be *None* anymore
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is now a bool only. Before the default was non and `.Artist.set_rasterized`
+documented to accept *None*. However, *None* did not have a special meaning
+and was treated as *False*.

lib/matplotlib/artist.py

@@ -113,7 +113,7 @@ def __init__(self):
         self._label = ''
         self._picker = None
         self._contains = None
-        self._rasterized = None
+        self._rasterized = False
         self._agg_filter = None
         # Normally, artist classes need to be queried for mouseover info if and
         # only if they override get_cursor_data.
@@ -898,7 +898,7 @@ def set_rasterized(self, rasterized):
 
         Parameters
         ----------
-        rasterized : bool or None
+        rasterized : bool
         """
         if rasterized and not hasattr(self.draw, "_supports_rasterization"):
             cbook._warn_external(