[auth0-js] add documentation for `username` signup option (#37363)
[auth0-js] add documentation for `username` signup option (#37363)

Update to #37259 and @auth0/auth0.js#1006
markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped