Document the append processor's media_type

[joegallo]

The set processor's media_type didn't document that there's a default value of "application/json", which I've fixed. More than that, though, the append processor also has a media_type that behaves the same way.

Neither of these things are new, they've both been this way more or less forever -- I don't know how backports and whatnot are handled on this repo, though. Can you help me with that, @pquentin?

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[joegallo]

I added some backport labels to cover all 9.x branches and the last 8.x branch. I don't know if what's ordinary for changes like this on this repo, though, so please tell me if we're should do something else.