Require explicit extern members in extern class/object and update docs

lynglib/src/jvmTest/kotlin/net/sergeych/lyng/miniast/CompletionEngineLightTest.kt

@@ -209,7 +209,7 @@ class CompletionEngineLightTest {
     fun inferredTypeFromMemberCall() = runBlocking {
         val code = """
             extern class MyClass {
-                fun getList(): List<String>
+                extern fun getList(): List<String>
             }
             extern val c: MyClass
             val x = c.getList()