Some final bits and a question for the trigger docs

Hi Bob,

Thanks for the updates to the trigger documentation! I think it's much better and more complete now. I made some small modifications to `content/devices/triggers.md` which are included in this pull request (among a major re-organization of the site, but that's not really relevant here). One thing that was not clear to me was the meaning of the `duration` field in the plug-in. Could you perhaps fill out that bit and submit the changes back to me? Feel free to make other changes as well, but this is the only thing that to me was unclear.

Then I will put the page online with the next major update of osdoc.

Thanks and cheers (and sorry for the slow response),
Sebastiaan