Style: do not highlight single return when as "Use expresison body"

idea/src/org/jetbrains/kotlin/idea/inspections/UseExpressionBodyInspection.kt

@@ -60,7 +60,7 @@ class UseExpressionBodyInspection(private val convertEmptyToUnit: Boolean) : Abs
         val toHighlight = valueStatement.toHighlight()
         return when {
             valueStatement !is KtReturnExpression -> Status(toHighlight, "block body", INFORMATION)
-            valueStatement.returnedExpression is KtWhenExpression -> Status(toHighlight, "'return when'", GENERIC_ERROR_OR_WARNING)
+            valueStatement.returnedExpression is KtWhenExpression -> Status(toHighlight, "'return when'", INFORMATION)
             valueStatement.isOneLiner() -> Status(toHighlight, "one-line return", GENERIC_ERROR_OR_WARNING)
             else -> Status(toHighlight, "return", INFORMATION)
         }

idea/testData/inspectionsLocal/useExpressionBody/when.kt

@@ -1,3 +1,5 @@
+// HIGHLIGHT: INFORMATION
+
 fun sign(x: Int): Int {
     <caret>return when {
         x < 0 -> -1

idea/testData/inspectionsLocal/useExpressionBody/when.kt.after

@@ -1,3 +1,5 @@
+// HIGHLIGHT: INFORMATION
+
 fun sign(x: Int): Int = when {
     x < 0 -> -1
     x > 0 -> 1