Document that `global_fetch_strictly_public` is required for skew protection

[oeed]

I went down a deep rabbit hole today trying to figure out why skew protection wasn't working for our worker. The worker to worker requests were returning 404. I stumbled across the global_fetch_strictly_public flag which fixed it. It'd be great for future users to document on the skew protection page that this is needed.

[vicb]

@oeed global_fetch_strictly_public is in our templates and docs but feel free to propose a change here, this is the right repo.
Thanks!