api_tx: updated comment on provided functions
api_tx: updated comment on provided functions

The functions api_tx:add_on_nr/2 and api_tx:add_del_on_list/3 where not reflected in the
documentation of api_tx.
digshock/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,seyyed/scalaris,digshock/scalaris,tectronics/scalaris,tectronics/scalaris,digshock/scalaris