Implement DurationUnit in JS, add doc summary and conversion test

libraries/stdlib/src/kotlin/time/DurationUnit.kt

@@ -1,6 +1,6 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
- * that can be found in the license/LICENSE.txt file.
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
 @file:kotlin.jvm.JvmMultifileClass()
@@ -9,6 +9,11 @@
 package kotlin.time
 
 
+/**
+ * The list of possible time measurement units, in which a duration can be expressed.
+ *
+ * The smallest time unit is [NANOSECONDS] and the largest is [DAYS], which corresponds to exactly 24 [HOURS].
+ */
 public expect enum class DurationUnit {
     /**
      * Time unit representing one nanosecond, which is 1/1000 of a microsecond.
@@ -35,12 +40,12 @@ public expect enum class DurationUnit {
      */
     HOURS,
     /**
-     * Time unit representing one day, which always equals 24 hours.
+     * Time unit representing one day, which is always equal to 24 hours.
      */
     DAYS;
 }
 
-
+/** Converts the given time duration [value] expressed in the specified [sourceUnit] into the specified [targetUnit]. */
 public expect fun convertDurationUnit(value: Double, sourceUnit: DurationUnit, targetUnit: DurationUnit): Double