net/http: try to document ServeFile security more
net/http: try to document ServeFile security more

We've expanded this several times. Try more.

Fixes #18837

Change-Id: I03b699391351a30ee60a15d7aa712c6c66444cf9
Reviewed-on: https://go-review.googlesource.com/125875
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,golang/go,nawawi/go,akutz/go,josharian/go,akutz/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,golang/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,akutz/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,josharian/go,cloudflare/go,josharian/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,josharian/go