Document NULL return value for Fl_Browser::find_line().

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@6854 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

src/Fl_Browser.cxx

@@ -144,7 +144,8 @@ const char *Fl_Browser::item_text(void *item) const {
   item_next(), etc. to access the internal linked list more efficiently.
 
   \param[in] line The line number of the item to return. (1 based)
-  \returns The returned item.
+  \retval item that was found.
+  \retval NULL if line is out of range.
   \see item_at(), find_line(), lineno()
 */
 FL_BLINE* Fl_Browser::find_line(int line) const {