Warn about event handlers

[gforcada]

At work we stumbled upon this: https://stackoverflow.com/questions/25443327

Which basically means: when editing an object its container does also gets notified about it, which could (depending on the event handler) cause a massive performance problem (let's say that you build some expensive pdfs, graphs, trigger reindexes or what not).

I guess that would be nice to let everyone know about it when reading the event handlers documentation, so here probably: https://5.docs.plone.org/develop/addons/components/events.html

Will try to send a pull request, no promise on it though (lack of time anyone? :S)