Clarify the meaning of the `STOP` button for runs

[LudovicoRighi]

People are confused about the differences among KILL, TERMINATE and STOP buttons for runs.

i.e. "we killed a run here and the state somehow transitioned into success"

It turns out they actually clicked STOP.

cc @gusostow

[alexhsamuel]

The meaning of "stop" is explained here: https://apsis-scheduler.readthedocs.io/en/latest/programs.html#program-stop
This is an abstraction; different program types and possibly different program instances may implement "stop" in different ways.

"KILL" and "TERMINATE", in contrast, send the corresponding Linux signal to the program process.

The run state this produces is also up to each program. The current programs (with defaults) generally set failure if the process terminates due to a signal, except for success if the run is already in stopping and it terminates due to SIGTERM.

[alexhsamuel]

I guess there's potentially a confusion that there is a SIGSTOP... though I suspect this is not the source of the confusion that was reported to you.

Maybe we should rename the KILL and TERMINATE operations to SIGKILL and SIGTERM, respectively?