Improve docs around append callback?

[villesau]

The docs indicates that append should have some sort of callback: https://www.archiverjs.com/docs/archiver#moduleprototypeappend

However, the syntax is not valid javascript and it's not clear how the callback should be defined and used. Could the documentation around that be clarified? I'd like to use it for debuggingand logging purposes.

[kbrownlees]

This for Format Registration FYI, not the normal append you would use (https://www.archiverjs.com/docs/archiver/#append).