DOC: add note about why we do extra work in this internal function

Author: tacaswell

lib/matplotlib/cm.py

@@ -702,5 +702,7 @@ def _ensure_cmap(cmap):
     if isinstance(cmap, colors.Colormap):
         return cmap
     cmap_name = cmap if cmap is not None else mpl.rcParams["image.cmap"]
+    # use check_in_list to ensure type stability of the exception raised by
+    # the internal usage of this (ValueError vs KeyError)
     _api.check_in_list(sorted(_colormaps), cmap=cmap_name)
     return mpl.colormaps[cmap_name]