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/observer/parse.go

@@ -10,6 +10,9 @@ import (
 	observer_plugin "github.com/go-gost/x/observer/plugin"
 )
 
+// ParseObserver converts an ObserverConfig into an observer.Observer. It only
+// supports plugin backends (HTTP or gRPC); returns nil when cfg or cfg.Plugin
+// is nil.
 func ParseObserver(cfg *config.ObserverConfig) observer.Observer {
 	if cfg == nil || cfg.Plugin == nil {
 		return nil