Add API change note.

Author: mdboom

doc/api/api_changes.rst

@@ -17,6 +17,16 @@ For new features that were added to matplotlib, please see
 Changes in 1.3.x
 ================
 
+Changes in 1.3.1
+----------------
+
+It is rare that we make an API change in a bugfix release, however,
+for 1.3.1 since 1.3.0 the following change was made:
+
+- `text.Text.cached` (used to cache font objects) has been made into a
+  private variable.  Among the obvious encapsulation benefit, this
+  removes this confusing-looking member from the documentation.
+
 Code removal
 ------------