code: public static StringDocument from(StringDocument doc, String id, String content) {
          return StringDocument.create(id, doc.expiry(), content, doc.cas(), doc.mutationToken());
      }
docs: Creates a copy from a different {@link StringDocument}, but changes the document ID and content.
  
  @param doc the original {@link StringDocument} to copy.
  @param id the per-bucket unique document id.
  @param content the content of the document.
  @return a copied {@link StringDocument} with the changed properties.