minor doc change for -tranche argument (#4517)

* minor doc change for -tranche argument

src/main/java/org/broadinstitute/hellbender/tools/walkers/vqsr/VariantRecalibrator.java

@@ -235,6 +235,7 @@ public class VariantRecalibrator extends MultiVariantWalker {
      * Add truth sensitivity slices through the call set at the given values. The default values are 100.0, 99.9, 99.0, and 90.0
      * which will result in 4 estimated tranches in the final call set: the full set of calls (100% sensitivity at the accessible
      * sites in the truth set), a 99.9% truth sensitivity tranche, along with progressively smaller tranches at 99% and 90%.
+     * Note: You must pass in each tranche as a separate value (e.g. -tranche 100.0 -tranche 99.9).
      */
     @Argument(fullName="truth-sensitivity-tranche",
             shortName="tranche",