Add references to available implementations in TestDispatcher docs (#3200)

Co-authored-by: dkhalanskyjb <[email protected]>

Author: zsmb13

kotlinx-coroutines-test/common/src/TestDispatcher.kt

@@ -10,6 +10,11 @@ import kotlin.jvm.*
 
 /**
  * A test dispatcher that can interface with a [TestCoroutineScheduler].
+ * 
+ * The available implementations are:
+ * * [StandardTestDispatcher] is a dispatcher that places new tasks into a queue.
+ * * [UnconfinedTestDispatcher] is a dispatcher that behaves like [Dispatchers.Unconfined] while allowing to control
+ *   the virtual time.
  */
 @ExperimentalCoroutinesApi
 public abstract class TestDispatcher internal constructor(): CoroutineDispatcher(), Delay {