DOC: Explain return value of seconadary_x/yaxis

Follow up to matplotlib#31405.

Author: timhoffm

lib/matplotlib/axes/_secondary_axes.py

@@ -329,6 +329,11 @@ def set_color(self, color):
 Returns
 -------
 ax : axes._secondary_axes.SecondaryAxis
+    The returned Axes is overlaid on top of the original Axes and all
+    components except for the complementary axis are hidden. You may modify
+    the complementary axis, e.g. by setting ticks or an axis label. However,
+    it is not designed to hold data, so that you should not call any
+    plotting methods on it.
 
 Other Parameters
 ----------------