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/src/OpenSSL/SSL.py b/src/OpenSSL/SSL.py
index 8ae7449..160cfd8 100644
--- a/src/OpenSSL/SSL.py
+++ b/src/OpenSSL/SSL.py
@@ -1590,11 +1590,12 @@
 
         _lib.SSL_set_shutdown(self._ssl, state)
 
-    def state_string(self):
+    def get_state_string(self):
         """
-        Get a verbose state description
+        Retrieve a verbose string detailing the state of the Connection.
 
         :return: A string representing the state
+        :rtype: bytes
         """
         return _ffi.string(_lib.SSL_state_string_long(self._ssl))