Add missing doc

Author: nicolasstucki

compiler/test/dotty/tools/dotc/FromTastyTests.scala

@@ -23,7 +23,7 @@ class FromTastyTests extends ParallelTesting {
   @Test def posTestFromTasty: Unit = {
     // Can be reproduced with
     // > sbt
-    // > dotc -Ythrough-tasty <source>
+    // > dotc -Ythrough-tasty -Ycheck:all <source>
 
     implicit val testGroup: TestGroup = TestGroup("posTestFromTasty")
     val (step1, step2) = compileTastyInDir("../tests/pos", defaultOptions,
@@ -60,7 +60,7 @@ class FromTastyTests extends ParallelTesting {
   @Test def runTestFromTasty: Unit = {
     // Can be reproduced with
     // > sbt
-    // > dotc -Ythrough-tasty <source>
+    // > dotc -Ythrough-tasty -Ycheck:all <source>
     // > dotr Test
 
     implicit val testGroup: TestGroup = TestGroup("runTestFromTasty")