Provide an example of using proto3's primitive wrappers.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=250656647

docs/bugpattern/ProtoFieldNullComparison.md

@@ -5,9 +5,9 @@ comparisons like these often indicate a nearby error.
 
 If you need to distinguish between an unset optional value and a default value,
 you have two options. In most cases, you can simply use the `hasField()` method.
-proto3 however does not generate `hasField()` methods for scalar fields of type
-`string` or `bytes`. In those cases you will need to wrap your field in
-`google.protobuf.StringValue` or `google.protobuf.BytesValue`, respectively.
+proto3 however does not generate `hasField()` methods for primitive types
+(including `string` and `bytes`). In those cases you will need to wrap your
+field in `google.protobuf.StringValue` or similar.
 
 NOTE: This check applies to normal (server) protos and Lite protos. The
 deprecated nano runtime does produce objects which use `null` values to indicate
@@ -40,3 +40,15 @@ void test(MyProto proto) {
   }
 }
 ```
+
+If the presence of a field is required information in proto3, the field can be
+wrapped. For example,
+
+```java {.good}
+message MyMessage {
+  google.protobuf.StringValue my_string = 1;
+}
+```
+
+Presence can then be tested using `myMessage.hasMyString()`, and the value
+retrieved using `myMessage.getMyString().getValue()`.