Workaround for multi-line outputs.

It seems that mdoc assumes a type name to be stringified into a
single line but that is wrong for RefinedType.

May be we should fix this line:
https://github.com/scalameta/mdoc/blob/69fea55c23477fbb6aed94e39b54b31efa56cf24/cli/src/main/scala/mdoc/internal/markdown/ReplVariablePrinter.scala#L25

site/src/main/resources/META-INF/services/mdoc.PostModifier

@@ -0,0 +1 @@
+site.MultilineOutputModifier

site/src/main/scala/site/MultilineOutputModifier.scala

@@ -0,0 +1,32 @@
+package site
+
+import mdoc.{PostModifier, PostModifierContext}
+
+class MultilineOutputModifier extends PostModifier {
+  val name = "mline"
+
+  def process(ctx: PostModifierContext): String = {
+    val code = ctx.variables.map { v =>
+      val s = v.toString
+      (s, linesAsComment(s))
+    }.foldLeft(ctx.outputCode) { case (code, (from, to)) =>
+        code.replace(from, to)
+    }
+
+    s"""```scala
+    |${code}
+    |```
+    |""".stripMargin
+  }
+
+  private def linesAsComment(lines: String): String =
+    lines.split("\n").map(lineAsComment(_)).mkString("\n")
+
+  private def lineAsComment(line: String): String =
+    if (line.isEmpty())
+      line
+    else if (line.startsWith("//"))
+      line
+    else
+      "// " + line
+}