net/http: document Server's implicit Content-Length response header
net/http: document Server's implicit Content-Length response header

Fixes #23450

Change-Id: I829399194299d2e6d5e754b60e8f72b321b5da90
Reviewed-on: https://go-review.googlesource.com/115040
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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