Merge pull request matplotlib#29969 from meeseeksmachine/auto-backport-of-pr-29965-on-v3.10.x

Backport PR matplotlib#29965 on branch v3.10.x (Document Axes.spines)

Author: QuLogic

doc/api/axes_api.rst

@@ -38,6 +38,7 @@ Attributes
 
    Axes.viewLim
    Axes.dataLim
+   Axes.spines
 
 Plotting
 ========

lib/matplotlib/axes/_base.py

@@ -566,6 +566,12 @@ class _AxesBase(martist.Artist):
     dataLim: mtransforms.Bbox
     """The bounding `.Bbox` enclosing all data displayed in the Axes."""
 
+    spines: mspines.Spines
+    """
+    The `.Spines` container for the Axes' spines, i.e. the lines denoting the
+    data area boundaries.
+    """
+
     xaxis: maxis.XAxis
     """
     The `.XAxis` instance.