Add xaxis and yaxis attributes to Axes docs

Author: timhoffm

doc/api/axes_api.rst

@@ -274,6 +274,8 @@ Axis / limits
    :template: autosummary.rst
    :nosignatures:
 
+   Axes.xaxis
+   Axes.yaxis
    Axes.get_xaxis
    Axes.get_yaxis
 

lib/matplotlib/axes/_base.py

@@ -827,10 +827,10 @@ def get_window_extent(self, renderer=None):
 
     def _init_axis(self):
         # This is moved out of __init__ because non-separable axes don't use it
-        self.xaxis = maxis.XAxis(self, clear=False)
+        self.xaxis = maxis.XAxis(self, clear=False)  #: The XAxis instance.
         self.spines.bottom.register_axis(self.xaxis)
         self.spines.top.register_axis(self.xaxis)
-        self.yaxis = maxis.YAxis(self, clear=False)
+        self.yaxis = maxis.YAxis(self, clear=False)  #: The YAxis instance.
         self.spines.left.register_axis(self.yaxis)
         self.spines.right.register_axis(self.yaxis)