[analyzer] Note that deadcode.UnmodifiedVariable would be an opt-in checker.

Website-only change.

llvm-svn: 188731

clang/www/analyzer/potential_checkers.html

@@ -297,6 +297,8 @@ void test() {
 <tr><td><span class="name">deadcode.UnmodifiedVariable
 <br>(C, C++)</span><br><br>
 A variable is never modified but was not declared const and is not a reference.
+<br><br>
+<i>(opt-in checker)</i>
 </td><td><pre>
 extern int computeDelta();