Mention Solo button in "Muting and Soloing"

Duplicate a couple of lines from "Other Toolbar Items" to put the information where it is needed.

include/muting-and-soloing.html

@@ -21,6 +21,11 @@
     busses which feed that bus.
   </li>
 </ul>
+<p>
+  The Solo status indicator button in the Toolbar blinks when one or more
+  tracks are being soloed. Clicking this button disables any active explicit
+  and implicit solo on all tracks and busses.
+</p>
 
 <h2>With a monitor bus</h2>
 <p>