os: document and emphasize a potential misuse of File.Fd
This CL revises the document of File.Fd that explicitly points
its user to runtime.SetFinalizer where contains the information
that a file descriptor could be closed in a finalizer and therefore
causes a failure in syscall.Write if runtime.KeepAlive is not invoked.
The CL also suggests an alternative of File.Fd towards File.SyscallConn.
Fixes #41505
Change-Id: I6816f0157add48b649bf1fb793cf19dcea6894b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/256899 Reviewed-by: Rob Pike <r@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>