DOC: Suggest replacement for tostring_rgb (matplotlib#25502)

* DOC: Suggest replacement

* DOC: Better doc

* DOC: Move

Author: larsoner

…xt_api_changes/deprecations/XXXXX-AL.rst …xt_api_changes/deprecations/25484-AL.rstdoc/api/next_api_changes/deprecations/XXXXX-AL.rst renamed to doc/api/next_api_changes/deprecations/25484-AL.rst doc/api/next_api_changes/deprecations/XXXXX-AL.rst renamed to doc/api/next_api_changes/deprecations/25484-AL.rst

@@ -1,3 +1,4 @@
 ``RendererAgg.tostring_rgb`` and ``FigureCanvasAgg.tostring_rgb``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-... are deprecated with no replacement.
+... are deprecated with no direct replacement. Consider using ``buffer_rgba``
+instead, which should cover most use cases.

lib/matplotlib/backends/backend_agg.py

@@ -280,7 +280,7 @@ def buffer_rgba(self):
     def tostring_argb(self):
         return np.asarray(self._renderer).take([3, 0, 1, 2], axis=2).tobytes()
 
-    @_api.deprecated("3.8")
+    @_api.deprecated("3.8", alternative="buffer_rgba")
     def tostring_rgb(self):
         return np.asarray(self._renderer).take([0, 1, 2], axis=2).tobytes()
 
@@ -412,7 +412,7 @@ def get_renderer(self):
             self._lastKey = key
         return self.renderer
 
-    @_api.deprecated("3.8")
+    @_api.deprecated("3.8", alternative="buffer_rgba")
     def tostring_rgb(self):
         """
         Get the image as RGB `bytes`.