Add doc comments on Conversion methods

Author: odersky

library/src/scala/Conversion.scala

@@ -17,10 +17,16 @@ package scala
 *  Also note that in bootstrapped dotty, `Predef.<:<` should inherit from
 *  `Conversion`. This would cut the number of special cases in `discardForView`
 *  from two to one.
+*
+*  The `Conversion` class can also be used to convert explicitly, using
+*  the `into` extension method.
 */
 @java.lang.FunctionalInterface
 abstract class Conversion[-T, +U] extends Function1[T, U]:
+  /** Convert value `x` of type `T` to type `U` */
   def apply(x: T): U
 object Conversion:
+
   extension [T](x: T)
+    /** `x.into[U]` converts a value `x` of type `T` to type `U` */
     def into[U](using c: Conversion[T, U]) = c(x)