Add docs for Config Server behavior when backend is not accessible

[csterwa]

The current Spring Cloud Config docs describe different configuration parameters for setting up a backend such as a Git repo. It would be helpful to describe expected behavior at runtime when the backend that was setup is not accessible for any number of reasons (network, crash, misconfiguration). Some of the situations to describe could be:

• What happens when Config Server starts up but is not able to access backend (Git)?
• What happens when backend is not accessible after Config Server already started successfully and pulled configuration values?
• What happens if backend becomes unavailable during process of pulling configuration values?
• How is refresh affected by backend becoming unaccessible?

I'm sure that there are other situations to be described. Please let me know if there are any questions. Thank you.