Edit description of Public/Protected/Private in new contest form to r…

…eflect current behaviour

"Protected" and "Private" actually have the same behaviour at the
moment (#107). Document the current behaviour until this is fixed.

app/views/contests/_form.html.erb

@@ -37,8 +37,8 @@
     Users can only join a contest if they have been added manually, or belong to a group with access to the contest.
     <ul>
         <li>Public: Anyone can see the contest.</li>
-        <li>Protected: Only those in one of the contest's groups can see the contest.</li>
-        <li>Private: Only those explicitly added as contestants can see the contest.</li>
+        <li>Protected: Only those who can join the contest can see the contest.</li>
+        <li>Private: Currently this is the same as "Protected", but this may be changed in future.</li>
     </ul>
 
   </div>