etcd.conf.yml.sample: improve discovery v3 configuration documentation

[xadips]

Update the sample configuration file to properly document discovery v3 options with correct nested structure and comprehensive comments.

Changes:

• Add properly structured discovery-config section with nested YAML
• Document all discovery v3 client configuration options
• Add default values in comments for timeout fields (in nanoseconds)
• Include important notes about duration format requirements
• Add security and authentication configuration options
• Clarify that only one of initial-cluster, discovery-srv, or discovery-config can be used
• Add CAUTION warning for insecure-skip-tls-verify option

This addresses the common misconfiguration issue where users attempt to use flat discovery-* fields at the top level, which are silently ignored in YAML config files. The flat fields only work as command-line flags, while YAML requires the nested discovery-config structure.

Fixes the documentation gap identified when investigating why discovery-token and discovery-endpoints were not being parsed from YAML configuration files.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

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[k8s-ci-robot]

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