add docs about auto-failure-recovery for failed nodes

[csuzhangxc]

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.

2. Describe your suggestion or addition.

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