Actually that ToolTip is already there:
Code:
<action name="actionCreateHTMLTOC">
<property name="text">
<string>&Create HTML Table Of Contents</string>
</property>
<property name="toolTip">
<string>Create a new HTML file using the current TOC.</string>
</property>
</action>
So there really is no change to the tooltip needed.
Changed menu entry to be "Create HTML from Table of Contents" to clarify things even further.