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/chain/parse.go

@@ -12,6 +12,9 @@ import (
 	"github.com/go-gost/x/registry"
 )
 
+// ParseChain converts a ChainConfig into a chain.Chainer. Each hop reference
+// is either looked up from the registry (by name) or parsed inline via
+// ParseHop. Returns nil with no error when cfg is nil.
 func ParseChain(cfg *config.ChainConfig, log logger.Logger) (chain.Chainer, error) {
 	if cfg == nil {
 		return nil, nil