bigquery: document that Schema field is optional
Fixes
@@ -541,6 +541,7 @@ func valuesToMap(vs []Value, schema Schema) (map[string]Value, error) {
type StructSaver struct {
// Schema determines what fields of the struct are uploaded. It should
// match the table's schema.
+ // Schema is optional for StructSavers that are passed to Uploader.Put.
Schema Schema
// If non-empty, BigQuery will use InsertID to de-duplicate insertions
