Improve documentation for passphrase callback (#703)

Address issue #701

doc/api/ssl.rst

@@ -369,8 +369,9 @@ Context objects have the following methods:
     this, it will be truncated.  Second, a boolean value which will be true if
     the user should be prompted for the passphrase twice and the callback should
     verify that the two values supplied are equal. Third, the value given as the
-    *userdata* parameter to :py:meth:`set_passwd_cb`.  If an error occurs,
-    *callback* should return a false value (e.g. an empty string).
+    *userdata* parameter to :py:meth:`set_passwd_cb`.  The *callback* must return
+    a byte string. If an error occurs, *callback* should return a false value
+    (e.g. an empty string).
 
 
 .. py:method:: Context.set_session_cache_mode(mode)

src/OpenSSL/SSL.py

@@ -695,7 +695,7 @@ def set_passwd_cb(self, callback, userdata=None):
         """
         Set the passphrase callback
 
-        :param callback: The Python callback to use
+        :param callback: The Python callback to use; must return a byte string
         :param userdata: (optional) A Python object which will be given as
                          argument to the callback
         :return: None