Improve docs for Consul HealthCheck vs HeartBeat

[TimHess]

See SteeltoeOSS/Steeltoe#1062 and SteeltoeOSS/Steeltoe#1245 for context, but the short version is we need to actually describe the difference between these two ways of keeping a service instance's status up to date in Consul and explain how to reliably choose which one is used and how it should be customized.

More info available in the Spring docs