[errorprone] util.Stats: Add summary fragment to javadoc

[errorprone] util.Stats: Add summary fragment to javadoc

Errorprone complains about missing summary in these javadocs.

[MissingSummary] A summary fragment is required; consider using the
value of the @return b lock as a summary fragment instead.

         * @return variance of the added values
           ^
(see https://google.github.io/styleguide/javaguide.html#s7.2-summary-fragment)
  Did you mean '*Returns variance of the added values.'?

Change-Id: I29d633ec95c18b17cc92b069dd1a94fbb2a75c94