code: public WebhookMessageBuilder append(String content)
      {
          Checks.notNull(content, "Content");
          Checks.check(this.content.length() + content.length() <= 2000,
              "Content may not exceed 2000 characters!");
          this.content.append(content);
          return this;
      }
docs: Appends to the currently set content of the resulting message.
  
  @param  content
  The content to append
  
  @throws java.lang.IllegalArgumentException
  If the provided content is {@code null} or
  the resulting content would exceed {@code 2000} characters in length
  
  @return The current WebhookMessageBuilder for chaining convenience