Add fine mode for using mouse scroll

include/using-the-mouse.html

@@ -118,6 +118,8 @@
 <p>
   Typically, scroll wheel input is used to adjust <dfn>continuous
   controls</dfn> such as faders and knobs, or to scroll vertically or
-  horizontally inside a window.
+  horizontally inside a window. In most <dfn>continuous control</dfn>
+  cases, holding down the <kbd>Ctrl</kbd> key while scrolling will use
+  "fine" mode and the scroll wheel increments will then be 10% of normal.
 </p>