DOCS: Serialization roundtrip removes mutability explanation (#2883)

* DOCS: Serialization roundtrip removes mutability explanation

* review comments

* review comments

* Review comments

* Review comments

* Review comments

node-api/src/test/kotlin/net/corda/nodeapi/internal/serialization/amqp/RoundTripTests.kt

@@ -0,0 +1,60 @@
+package net.corda.nodeapi.internal.serialization.amqp
+
+import net.corda.core.serialization.ConstructorForDeserialization
+import org.assertj.core.api.Assertions
+import org.junit.Test
+
+class RoundTripTests {
+    @Test
+    fun mutableBecomesImmutable() {
+        data class C(val l : MutableList<String>)
+        val factory = testDefaultFactoryNoEvolution()
+        val bytes = SerializationOutput(factory).serialize(C(mutableListOf ("a", "b", "c")))
+        val newC = DeserializationInput(factory).deserialize(bytes)
+
+        Assertions.assertThatThrownBy {
+            newC.l.add("d")
+        }.isInstanceOf(UnsupportedOperationException::class.java)
+    }
+
+    @Test
+    fun mutableStillMutable() {
+        class C {
+            val l : MutableList<String>
+
+            @Suppress("Unused")
+            constructor (l : MutableList<String>) {
+                this.l = l.toMutableList()
+            }
+        }
+        val factory = testDefaultFactoryNoEvolution()
+        val bytes = SerializationOutput(factory).serialize(C(mutableListOf ("a", "b", "c")))
+        val newC = DeserializationInput(factory).deserialize(bytes)
+
+        newC.l.add("d")
+    }
+
+    @Test
+    fun mutableStillMutable2() {
+        data class C (val l : MutableList<String>){
+            @ConstructorForDeserialization
+            @Suppress("Unused")
+            constructor (l : Collection<String>) : this (l.toMutableList())
+        }
+
+        val factory = testDefaultFactoryNoEvolution()
+        val bytes = SerializationOutput(factory).serialize(C(mutableListOf ("a", "b", "c")))
+        val newC = DeserializationInput(factory).deserialize(bytes)
+
+        newC.l.add("d")
+    }
+
+    @Test
+    fun mutableBecomesImmutable4() {
+        data class C(val l : List<String>)
+        val factory = testDefaultFactoryNoEvolution()
+        val bytes = SerializationOutput(factory).serialize(C(listOf ("a", "b", "c")))
+        val newC = DeserializationInput(factory).deserialize(bytes)
+        val newC2 = newC.copy (l = (newC.l + "d"))
+    }
+}