doc: clarify that v8.serialize() is not deterministic

Fixes: nodejs/help#2448

Author: addaleax

doc/api/v8.md

@@ -268,7 +268,9 @@ if (isMainThread) {
 
 The serialization API provides means of serializing JavaScript values in a way
 that is compatible with the [HTML structured clone algorithm][].
+
 The format is backward-compatible (i.e. safe to store to disk).
+Equal JavaScript values may result in different serialized output.
 
 ### `v8.serialize(value)`
 <!-- YAML