diff --git a/OpenSSL/_util.py b/OpenSSL/_util.py
index da8270b..bb2b740 100644
--- a/OpenSSL/_util.py
+++ b/OpenSSL/_util.py
@@ -59,6 +59,18 @@
 )
 
 def warn_text(label, obj):
+    """
+    If ``obj`` is text, emit a warning that it should be bytes instead and try
+    to convert it to bytes automatically.
+
+    :param str label: The name of the parameter from which ``obj`` was taken
+        (so a developer can easily find the source of the problem and correct
+        it).
+
+    :return: If ``obj`` is the text string type, a ``bytes`` object giving the
+        UTF-8 encoding of that text is returned.  Otherwise, ``obj`` itself is
+        returned.
+    """
     if isinstance(obj, text_type):
         warn(
             _TEXT_WARNING.format(label),