diff --git a/src/LyXAction.cpp b/src/LyXAction.cpp
index 01095ea353..b8120d6c36 100644
--- a/src/LyXAction.cpp
+++ b/src/LyXAction.cpp
@@ -1612,8 +1612,9 @@ void LyXAction::init()
  * \li Action: In text mode enters math mode (i.e. puts math insets on the current
                cursor position), in math mode enters text mode inside math expression.
  * \li Notion: If there is some selected text, it puts the text inside created math box.
- * \li Syntax: math-mode [<ARG>]
- * \li Params: <ARG>: eventual argument (LaTeX code) is passed to #LFUN_MATH_INSERT .
+ * \li Syntax: math-mode [on|<ARG>]
+ * \li Params: <ARG>: eventual argument (LaTeX code) is passed to #LFUN_MATH_INSERT .\n
+                "on": allow only entering of math mode from text mode.
  * \li Origin: Alejandro, 4 Jun 1996
  * \endvar
  */