Document the callback function for sendError (#483)
Document the callback function for sendError (#483)

Updating the docs to reflect the new situation for the `sendError` and
`wrap` helpers changed in PR:
https://github.com/appsignal/appsignal-javascript/pull/514

I could not find documentation on how the—now deprecated—`tags` and
`namespace` parameters work. It appears to not have been documented
previously, so I'm not adding it here either, since they are now
deprecated.

Part of https://github.com/appsignal/integration-guide/issues/41
appsignal/appsignal-docs