man: delete note about propagating signal termination

man: delete note about propagating signal termination

That advice is generally apropriate for "user" programs, i.e. programs which
are run interactively and used pipelines and such. But it makes less sense for
daemons to propagate the exit signal. For example, if a process receives a SIGTERM,
it is apropriate for it to exit with 0 code. So let's just delete the whole
paragraph, since this page doesn't seem to be the right place for the longer
discussion which would be required to mention all the caveats and considerations.

Fixes #6415.