Discourage alternate strings for 'none' linestyle

Inspired by but independent of the discussion around the representation of linestyles matplotlib#29304.

For other linestyles we have a short visual representation (e.g. "--") and a name (dashed "dashed"). No linestyle currently has four accepted writings "", " ", "none", "None".

This PR recommends only to use "" (visual representation) and "none" (named representation). It discourages " " and "None".  This should guide users towards more canonical usage. We may later decide whether to deprecate the alternatives.

Note also, that we have not used the alternatives in examples.

Author: timhoffm

lib/matplotlib/lines.py

@@ -1152,15 +1152,15 @@ def set_linestyle(self, ls):
 
             - A string:
 
-              ==========================================  =================
-              linestyle                                   description
-              ==========================================  =================
-              ``'-'`` or ``'solid'``                      solid line
-              ``'--'`` or  ``'dashed'``                   dashed line
-              ``'-.'`` or  ``'dashdot'``                  dash-dotted line
-              ``':'`` or ``'dotted'``                     dotted line
-              ``'none'``, ``'None'``, ``' '``, or ``''``  draw nothing
-              ==========================================  =================
+              ========================================================  ================
+              linestyle                                                 description
+              ========================================================  ================
+              ``'-'`` or ``'solid'``                                    solid line
+              ``'--'`` or  ``'dashed'``                                 dashed line
+              ``'-.'`` or  ``'dashdot'``                                dash-dotted line
+              ``':'`` or ``'dotted'``                                   dotted line
+              ``' '`` or ``'none'`` (discouraged: ``'None'``, ``' '``)  draw nothing
+              ========================================================  ================
 
             - Alternatively a dash tuple of the following form can be
               provided::

lib/matplotlib/patches.py

@@ -445,15 +445,15 @@ def set_linestyle(self, ls):
         """
         Set the patch linestyle.
 
-        ==========================================  =================
-        linestyle                                   description
-        ==========================================  =================
-        ``'-'`` or ``'solid'``                      solid line
-        ``'--'`` or  ``'dashed'``                   dashed line
-        ``'-.'`` or  ``'dashdot'``                  dash-dotted line
-        ``':'`` or ``'dotted'``                     dotted line
-        ``'none'``, ``'None'``, ``' '``, or ``''``  draw nothing
-        ==========================================  =================
+        ========================================================  ================
+        linestyle                                                 description
+        ========================================================  ================
+        ``'-'`` or ``'solid'``                                    solid line
+        ``'--'`` or  ``'dashed'``                                 dashed line
+        ``'-.'`` or  ``'dashdot'``                                dash-dotted line
+        ``':'`` or ``'dotted'``                                   dotted line
+        ``' '`` or ``'none'`` (discouraged: ``'None'``, ``' '``)  draw nothing
+        ========================================================  ================
 
         Alternatively a dash tuple of the following form can be provided::