doc(storage): clarify metadata restrictions
This is a tricky bit related to the fact that the Go client uses
the XML API for downloads. If the user defined metadata fields
with these characters, downloads fail in a confusing way.
Fixes
@@ -1312,6 +1312,10 @@ type ObjectAttrs struct {
// Metadata represents user-provided metadata, in key/value pairs.
// It can be nil if no metadata is provided.
+ //
+ // For object downloads using Reader, metadata keys are sent as headers.
+ // Therefore, avoid setting metadata keys using characters that are not valid
+ // for headers. See https://www.rfc-editor.org/rfc/rfc7230#section-3.2.6.
Metadata map[string]string
// Generation is the generation number of the object's content.
