mqtt: don't send items between < and >

The dashboard I'm using, node-red, and I'm sure others will take a payload text and re-display it as is. That means anything between < and > would be seen as an HTML tag and not displayed. In an ideal world it should probably change to < and > but not the case. Rather than everyone having to do that, it seems like "paging sysop" might be on the only place I've seen it.

Node 1 paged sysop for chat

If this isn't going to be changed then we should probably note it in the wiki so that others don't get caught out wondering where their text disappeared to.