Document how to use old images with embedded text

With all the changes in #868942: Redo image replacement handling there's bound to be people who want to know how to use their old images with text embedded. I'm breaking that into a separate issue. Here is a suggestion from meustrus carried over from that issue:

a.reply-button { 
  display: inline-block; 
  width: 99px; 
  height: 17px; 
  background: url(path/to/image); 
} 

a.reply-button span { 
  position: relative; 
  left: 9999px; 
}

This will end up in the handbook rather than as part of the module itself because it's just too messy to try and carry over legacy image handling. This isn't something that I need since I converted to the new style so I'd appreciate it if someone who needs this would test it out and make sure it works for you.

Michelle