docs(parsing): add package and symbol doc comments

Add godoc comments for all 62 exported symbols across 20 files in
config/parsing/: 23 MDKey constants, 35 Parse*/List/Default* functions,
3 TLS helpers, 1 Args struct with 7 fields, and the package doc.

config/parsing/sd/parse.go

@@ -10,6 +10,8 @@ import (
 	sd_plugin "github.com/go-gost/x/sd/plugin"
 )
 
+// ParseSD converts an SDConfig into an sd.SD. It only supports plugin backends
+// (HTTP or gRPC); returns nil when cfg or cfg.Plugin is nil.
 func ParseSD(cfg *config.SDConfig) sd.SD {
 	if cfg == nil || cfg.Plugin == nil {
 		return nil