Jerryzz0 patch 1

[jerryzz0]

Mention that Fetch Metadata policies requires setting the Vary header

[NDevTK]

I think a hint box should be used here https://xsleaks.dev/docs/contributions/ There's also a message about this in https://xsleaks.dev/docs/attacks/navigations/#partitioned-http-cache-bypass

Maybe tell people why the header is needed and why it's not the default behavior.

[terjanq]

+1 to what @NDevTK suggested. Could you make those changes?