It would look like this:
@returns {M} <code>this</code>
{M}
indicates the class of what is returned, and <code>this</code>
clearly indicates what is returned. If you use the markdown plugin:
@returns {M} `this`
A more verbose version would be:
@returns {M} The instance on which this method was called.
Or you could omit the class information and let the reader infer it.