fix comment for SetExpiresAfter config

Author: krichprollsch

config.go

@@ -60,8 +60,8 @@ func (c *SignConfig) SetExpires(expires int64) *SignConfig {
 	return c
 }
 
-// SetExpiresAfter adds an "expires" parameter containing an expiration
-// deadline after the given delay, as Unix time.
+// SetExpiresAfter sets the "expires" parameter to createdTime + delay (seconds).
+// Use this for a relative validity window instead of an absolute timestamp.
 // Default: 0 (do not add the parameter).
 func (c *SignConfig) SetExpiresAfter(delay int64) *SignConfig {
 	c.expiresAfter = delay