docs(oauth2): note single audience per ID-JAG draft spec

Signed-off-by: kanywst <niwatakuma@icloud.com>

Author: kanywst

server/oauth2.go

@@ -304,6 +304,7 @@ func (s *Server) newAccessToken(ctx context.Context, clientID string, claims sto
 const idJAGTyp = "oauth-id-jag+jwt"
 
 // idJAGClaims is the JWT payload for an ID-JAG token.
+// Audience is a single string per draft-ietf-oauth-identity-assertion-authz-grant-02.
 type idJAGClaims struct {
 	Issuer   string `json:"iss"`
 	Subject  string `json:"sub"`