added open-in-new-tab docs

svn: r17142

collects/scribblings/tools/unit.scrbl

@@ -575,6 +575,11 @@ Returns the currently active tab.
 
 }
                                           
+@defmethod[(open-in-new-tab [filename (or/c path-string? #f)]) void?]{
+  Opens a new tab in this frame. If @scheme[filename] is a @scheme[path-string?],
+  It loads that file in the definitions window of the new tab.
+}
+
 @defmethod[#:mode public-final (close-current-tab) void?]{
   Closes the current tab, making some other tab visible.
   If there is only one tab open, this method does nothing.