docs: add godoc comments to all exported functions and methods

mc/mapstructure/default.go

@@ -2,6 +2,7 @@ package mapstructure
 
 var DefaultFormat = NewFormat("mapstructure", WithRequireAll(), WithTrySnakeCase(), WithTryLowCase())
 
+// Decode is a stub placeholder. Use Format.Decode instead.
 func Decode(data any, v any) error {
 	return nil
 }