Etcd: docs: include mention of etcd configuration yaml in configuration.md

[philips]

Configuration.md doesn't mention the etcd.conf file format or the order of precedent between flags, file, and environment variables.

[xiang90]

@spzala

might want to pick it up?

[gyuho]

Currently, if etcd is configured with config file, environmental values are ignored

https://github.com/coreos/etcd/blob/2f57a84ee6294e2d1e3dcda6ef2388bc582a9844/etcdmain/config.go#L247-L255

[spzala]

@xiang90 sure, sounds good, I will be working on it. Thanks!

[spzala]

WIP. I also noticed that some flags are in the Configuration.md are not in the etcd.conf (e.g. metrics, listen-metrics-urls etc) and vice-versa (e.g. log-output is missed in the Configuration.md). I will add them as part of the fix unless there was a specific reason. Also, I wonder if it makes sense to have flags ordered alphabetically (per section)? Thanks!

[xiang90]

@spzala

Also, I wonder if it makes sense to have flags ordered alphabetically (per section)?

Some of them might be grouped again within the section. For example, we put all network related flags together again. But it seems that it is man's convention to sort them alphabetically. No strong opinion here.

[spzala]

@xiang90 thanks, makes sense, so let's not worry about that part under this issue. May be something we can talk about as part of future discussions.