os: document error returned by pending I/O operations on Close

Currently, File.Close only documents that "an" error will be returned
by pending I/O operations. Update the documentation to say that error
is specifically ErrClosed.

Change-Id: Ica817c9196ad6cb570c826789d37a4ff15a5d13d
Reviewed-on: https://go-review.googlesource.com/c/go/+/371015


Trust: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>