Improve documentation on `refresh` health indicator

[mlichtblau]

Currently the documentation does not mention that a "refresh" health indicator RefreshScopeHealthIndicator is added.
Should this information be added and also instructions how to disable it similar to the DiscoveryClient documentation.

[ryanjbaxter]

Sounds worthwhile to me

[seonghyeoklee]

Hi, I'd like to work on this issue. I'll add documentation for the RefreshScopeHealthIndicator and instructions on how to disable it. Will submit a PR soon.