[RESUBMIT]Compatibility of dubbo.registry.address/dubbo.registries.address

- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.x
* Java version: 1.8

### Steps to reproduce this issue
related to #1725

### Expected Result
In legacy document it supports format like `addr1|addr2`.

### Actual Result
Throw an exception during booting.

## Update
It seems that it's quite an old issue for a long time. So we should decide if we should **fix it** or **drop the feature** and update the document (Especially in the upgrade guide line).