Rename state_name to get_state_name

A bit more consistent naming.  Rename is possible because the method
hasn't been part of a release yet.
diff --git a/doc/api/ssl.rst b/doc/api/ssl.rst
index e3fe8c9..d144cfd 100644
--- a/doc/api/ssl.rst
+++ b/doc/api/ssl.rst
@@ -770,10 +770,7 @@
     BIO.
 
 
-.. py:method:: Connection.state_string()
-
-    Retrieve a verbose string detailing the state of the Connection.
-
+.. automethod:: Connection.get_state_string
 
 .. py:method:: Connection.client_random()