Add optional callback description on sync method

[jpdemagalhaes]

The sync method also accepts an optional callback (just as the attach method), but there wasn't any indicators of this possibility on the docs, hence, this PR.

This adds a description and an example on the docs, hopefully, giving devs more clarity on what's available 😄

[jpdemagalhaes]

@RomainLanz any chance you can merge this one?

Or is there anything else to be done on this PR?

[victorLessa]

Does this callback return the array index as a parameter too?