Ajax: replace "jqXHR.complete" callback with "always"
Ajax: replace "jqXHR.complete" callback with "always"

Since it was deprecated since 1.8.

Also add additional comments which explains tricky
behaviour of "always" callback

Closes gh-2033

roytoo/jquery,jquery/jquery,roytoo/jquery,jquery/jquery,jquery/jquery,roytoo/jquery