document signal severity

in `uninstall` stanzas
No, `INT` is removed and demoted down the list, unless I'm missing something.

In theory, you only want to send the least-rude signal which also works.  In practice,
`:signal` only exists for hard cases where `:quit` didn't work, so I expect there will always
be hackery here such as sending multiple signals.

Also, `uninstall` stops sending signals as soon as the process disappears.  I think I should
add that to this PR.