Document in-out param of dhm_calc_secret()
diff --git a/include/polarssl/dhm.h b/include/polarssl/dhm.h
index 48d9268..0a32bd9 100644
--- a/include/polarssl/dhm.h
+++ b/include/polarssl/dhm.h
@@ -218,7 +218,8 @@
*
* \param ctx DHM context
* \param output destination buffer
- * \param olen number of chars written
+ * \param olen on entry, must hold the size of the destination buffer
+ * on exit, holds the actual number of bytes written
*
* \return 0 if successful, or an POLARSSL_ERR_DHM_XXX error code
*/