Fix: add documentation

Author: alexander-myltsev

src/dotty/tools/dotc/transform/ClassOf.scala

@@ -9,6 +9,11 @@ import core.Symbols.TermSymbol
 import core.TypeErasure
 import TreeTransforms.{MiniPhaseTransform, TransformerInfo, TreeTransform}
 
+/** Performs rewritings as follows for `classOf` calls:
+ * classOf[CustomValueClass] ~> CustomValueClass type
+ * classOf[ValueClass]       ~> ValueClass type, where ValueClass is Boolean, Byte, Short, etc.
+ * classOf[AnyOtherType]     ~> erasure(AnyOtherType)
+ */
 class ClassOf extends MiniPhaseTransform {
   import tpd._