Genericise recipes

[kfrn]

Something raised in #429 (sparked by comments here #422 (comment) and here #428 (comment))
was that a bunch of existing recipes could be elaborated and genericised

• rewritten to use placeholder variables (this is preferable), and/or
• fuller explanations given to any hardcoded variables

An example of this is the Fade audio & video streams recipe.

I'm interested in having a look over the recipes, but it might not be soon, so anyone is welcome to pick this up!

[kieranjol]

Thanks Katherine!