Implement correct is check for SuspendFunction

Add test for suspend functions with language version 1.2.
Minor edit in SuspendFunction docs.

compiler/testData/codegen/box/coroutines/featureIntersection/suspendFunction12.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND: JS_IR, JS
+// IGNORE_BACKEND: JVM_IR
+// WITH_RUNTIME
+// WITH_COROUTINES
+// LANGUAGE_VERSION: 1.2
+
+import helpers.*
+import kotlin.coroutines.*
+
+val lambda1 = { x: Any -> } as (Any) -> Unit
+val suspendLambda0: suspend () -> Unit = {}
+
+fun box(): String {
+    assert(lambda1 is SuspendFunction0<*>) { "Failed: lambda1 !is SuspendFunction0<*>" }
+    assert(suspendLambda0 is Function1<*, *>) { "Failed: suspendLambda0 is Function1<*, *>" }
+    assert(suspendLambda0 is SuspendFunction0<*>) { "Failed: suspendLambda0 is SuspendFunction0<*>" }
+
+    return "OK"
+}