Add docs to TestREPL

Author: odersky

test/test/TestREPL.scala

@@ -5,6 +5,14 @@ import dotty.tools.dotc.core.Contexts.Context
 import collection.mutable
 import java.io.StringWriter
 
+/** A subclass of REPL used for testing.
+ *  It takes a transcript of a REPL session in `script`. The transcript
+ *  starts with the first input prompt `scala> ` and ends with `scala> :quit` and a newline.
+ *  Invoking `process()` on the `TestREPL` runs all input lines and
+ *  collects then interleaved with REPL output in a string writer `out`.
+ *  Invoking `check()` checks that the collected output matches the original
+ *  `script`.
+ */
 class TestREPL(script: String) extends REPL {
 
   private val out = new StringWriter()