Clarify the purpose of the message parameter of assertFailsWith

#KT-22869 Fixed

libraries/kotlin.test/js/src/main/kotlin/kotlin/test/JsImpl.kt

@@ -17,7 +17,11 @@ actual fun todo(block: () -> Unit) {
 }
 
 
-/** Asserts that a [block] fails with a specific exception of type [exceptionClass] being thrown. */
+/**
+ * Asserts that a [block] fails with a specific exception of type [exceptionClass] being thrown.
+ *
+ * If the assertion fails, the specified [message] is used unless it is null as a prefix for the failure message.
+ */
 actual fun <T : Throwable> assertFailsWith(exceptionClass: KClass<T>, message: String?, block: () -> Unit): T {
     val exception = assertFails(message, block)
     @Suppress("INVISIBLE_MEMBER")