Fix ModifiedButNotUsed doc

The code snippet was calling ".toBuilder" on something that was already a Builder (which I’m 99% sure does not compile?).

The documentation "or the Builder modified in place" which sounds to me like it implies it is a preferred solution. So should be tagged as ".good".

Finally, renamed "setFoo" to "withFoo", as that seems like the convention for functions that "return a modified copy, but don’t touch input". Added "@CheckReturnValue" to drive home the point that if you are calling this method, you still need to *also* use the returned value.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=231218053

docs/bugpattern/ModifiedButNotUsed.md

@@ -20,18 +20,19 @@ something is done with the return value:
   }
 ```
 
-As protos are immutable, the return value must be used:
+As protos are immutable, either the return value must be used:
 
 ```java {.good}
-  MyProto setFoo(MyProto proto, String foo) {
+  @CheckReturnValue
+  MyProto withFoo(MyProto proto, String foo) {
     return proto.toBuilder().setFoo(foo).build();
   }
 ```
 
 or the Builder modified in place:
 
-```java {.bad}
+```java {.good}
   void setFoo(MyProto.Builder protoBuilder, String foo) {
-    protoBuilder.toBuilder().setFoo(foo);
+    protoBuilder.setFoo(foo);
   }
 ```