Update json-last-error.xml to mention validation (#4722)

Author: juan-morales

reference/json/functions/json-last-error.xml

@@ -13,7 +13,7 @@
    <void />
   </methodsynopsis>
   <para>
-   Returns the last error (if any) occurred during the last JSON encoding/decoding,
+   Returns the last error (if any) occurred during the last JSON validation/encoding/decoding,
    which did not specify <constant>JSON_THROW_ON_ERROR</constant>.
   </para>
  </refsect1>