os: consolidate and clarify File.Fd docs

os: consolidate and clarify File.Fd docs

Change-Id: Id062b969fe7d6908a0797b36a4a379e4d46ba557
Reviewed-on: https://go-review.googlesource.com/c/go/+/648516
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>