Add to TexStudio autocompletion

For completeness: You find more info on how the completer works in the FAQ.

In the special case of eqnarray*, it's not really missing but included in l2tabu.cwl. This is not loaded by default, because eqnarray* is deprecated. If you insist to use it for some reason, you can include it by checking l2tabu.cwl at Options -> Configure -> Completion -> Use following completion files.