docs: clarify limitation around reverse_proxy

[dbaynard]

Simple change, would have saved me multiple hours of work. I assume Reverse_proxy with custom http payload - Help - Caddy Community still holds.

That[every proxy request modifying the payload]’s out of scope of reverse_proxy.

You should write your own module to do this.

Edit: I've added a further, related commit. Again, hours of work.