document unroll annot

Author: bishabosha

library/src/scala/annotation/unroll.scala

@@ -1,4 +1,12 @@
 package scala.annotation
 
 @experimental("under review as part of SIP-61")
+/**The `@unroll` annotation is reserved for parameters of classes and methods.
+ *
+ * It enables to add new parameters while preserving backwards binary compatibility,
+ * through code generation of hidden forwarder methods (but visible in the binary API).
+ *
+ * Read more about parameter unrolling, and the usage of `@unroll` in the reference documentation:
+ * https://dotty.epfl.ch/docs/reference/experimental/unrolled-defs.html
+ */
 final class unroll extends scala.annotation.StaticAnnotation