Adding doc comment.

Author: lahodaj

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/ExhaustivenessComputer.java

@@ -47,7 +47,8 @@
 
 import static java.util.stream.Collectors.groupingBy;
 
-/**
+/** A class to compute exhaustiveness of set of switch cases.
+ *
  *  <p><b>This is NOT part of any supported API.
  *  If you write code that depends on this, you do so at your own risk.
  *  This code and its internal interfaces are subject to change or