os: further document limitations around naked file descriptors

NewFile requires the file descriptor to be either closed
through the returned File instance, or to stay valid at least
until the finalizer runs during garbage collection.

These requirements are easily violated when file descriptors
are closed via unix.Close, or when the *File returned by
NewFile is garbage collected while the underlying file descriptor is
still in use.

This commit adds further documentation for NewFile and Fd, making it
explicit that using naked file descriptors is subject to constraints
due to garbage collection of File objects.

Fixes #43863

Change-Id: I49ea1f0054eb2d2a72b616450c8e83476f4d07fb
GitHub-Last-Rev: 180d0130ae9009456914fb265b4bafa0e599de0e
GitHub-Pull-Request: golang/go#43867
Reviewed-on: https://go-review.googlesource.com/c/go/+/286032


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>