Update comment

Author: odersky

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -512,8 +512,11 @@ trait ConstraintHandling {
    *   1. If `inst` is a singleton type, or a union containing some singleton types,
    *      widen (all) the singleton type(s), provided the result is a subtype of `bound`.
    *      (i.e. `inst.widenSingletons <:< bound` succeeds with satisfiable constraint)
-   *   2. If `inst` is a union type, approximate the union type from above by an intersection
-   *      of all common base types, provided the result is a subtype of `bound`.
+   *   2a. If `inst` is a union type and `widenUnions` is true, approximate the union type
+   *      from above by an intersection of all common base types, provided the result
+   *      is a subtype of `bound`.
+   *   2b. If `inst` is a union type and `widenUnions` is false, turn it into a hard
+   *      union type (except for unions | Null, which are kept in the state they were).
    *   3. Widen some irreducible applications of higher-kinded types to wildcard arguments
    *      (see @widenIrreducible).
    *   4. Drop transparent traits from intersections (see @dropTransparentTraits).