The current helptext for listtransactions, listsinceblock and gettransaction only list two of the five possible options for category. This incorrectly implies that these are the only two options, and can cause problems if the other three options aren't accounted for. Also, some of the documentation is incorrect when specifying which options are returned for which categories.
This PR updates the helptext for these RPCs and adds a functional regression test for the cases when the other three categories are returned.