DOC: Clarify y paramter in Axes.set_title

Autopositioning only works if rcParams["axes.titley"] is None in addition
to y being None in the function call. If rcParams["axes.titley"] is set
to a float, there's no way to enforce autopositioning by function call
parameters (passing y=None has no effect in this case).

Author: StefRe

lib/matplotlib/axes/_axes.py

@@ -119,8 +119,8 @@ def set_title(self, label, fontdict=None, loc=None, pad=None, *, y=None,
 
         y : float, default: :rc:`axes.titley`
             Vertical Axes loation for the title (1.0 is the top).  If
-            None (the default), y is determined automatically to avoid
-            decorators on the Axes.
+            None (the default) and :rc:`axes.titley` is also None, y is
+            determined automatically to avoid decorators on the Axes.
 
         pad : float, default: :rc:`axes.titlepad`
             The offset of the title from the top of the Axes, in points.