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

lynglib/src/commonTest/kotlin/BindingTest.kt

@@ -164,7 +164,7 @@ class BindingTest {
         val ms = Script.newScope()
         ms.eval("""
             extern class A {
-                fun get1(): String
+                extern fun get1(): String
             }
             
             extern fun getA(): A
@@ -184,4 +184,3 @@ class BindingTest {
     }
 }
 
-