Document incoming headers and body

[nickvergessen]

Similarly to https://github.com/nextcloud/spreed/blob/eca3161d246eac7e874a17ee3b747bb0129bee7b/docs/recording.md?plain=1#L120-L125

[provokateurin]

What about adding an attribute for request headers? I don't think we can implement this in some way that is possible to validate by psalm.

As for the request body it's more complicated, as it should be possible to use the doc types to describe it, but that is not possible within an attribute.