Strengthen the text about not taking on-stage Q&A.

The text is from #827.

Fixes #1256, but doesn't include the "is audience interactive" checkbox.

templates/emails/cfp-slot-scheduled.txt

@@ -21,7 +21,7 @@ Things to note:
 * You can also update your {{ proposal.human_type }} title and description at any time using the link above, so make sure it sounds great!
 
 {% if proposal.type == 'talk' %}
-* While you are allowed on-stage Q&A within your slot, we would prefer it if you asked the audience to meet you somewhere afterwards to discuss it.
+* We would strongly prefer that you do not take on-stage Q&A after your talk unless it is, for instance, a panel that requires audience participation - these are hard for us to manage, and frequently unproductive. Please consider taking questions outside the tent, or in the bar instead.
 {% endif %}
 
 * All this information and more is available on our speakers website page: https://wiki.emfcamp.org/wiki/Speaking