add description about `spec.enableAlertRules` and `spec.alertmanagerURL` about TidbMonitor

[KanShiori]

Change Request

This repository is ONLY used to solve issues related to TiDB Operator documentation. For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

TidbMonitor don't add rule files to Prometheus by default, unless set spec.enableAlertRules and spec.alertmanagerURL

2. Describe your suggestion or addition.

3. Provide some reference materials (documents, websites, etc) if you could.

[shichun-0415]

@KanShiori Could you help file a PR to fix this issue? Thanks.