clarify comment in useCompanionAsSumMirror

Author: bishabosha

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -116,7 +116,12 @@ object SymUtils:
       self.linkedClass.exists
       && !self.is(Scala2x)
       && (
-        // self is from source, or companion is a subtype of Sum
+        // If the sum type is compiled from source, and `self` is a "generic sum"
+        // then its companion object will become a sum mirror in `posttyper`. (This method
+        // can be called from `typer` when summoning a Mirror.)
+        // However if `self` is from a prior run then we should check that its companion subclasses `Mirror.Sum`.
+        // e.g. before Scala 3.1, hierarchical sum types were not considered "generic sums", so their
+        // companion would not cache the mirror. Companions from TASTy will already be typed as `Mirror.Sum`.
         self.isDefinedInCurrentRun
         || self.linkedClass.isSubClass(defn.Mirror_SumClass)
       )