Update assignEntities documentation to state max users per call

The admin.auth.policy.assignEntities page does not have any language on how many users you can add in one go. According to Slack Support ticket number 6964048, the maximum amount of users you can add to a single call is 1,000. It would be amazing if the documentation could note this.

The page URLs

• https://docs.slack.dev/reference/methods/admin.auth.policy.assignEntities/

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

打开链接的 admin.auth.policy.assignEntities 参考页面，检查其中当前关于在一次调用中添加用户的措辞。完成的标准是页面明确说明单次调用的用户上限为 1,000 人；提交前请遵循链接的贡献指南。