doc: clarify safety of multiple and concurent os.(*File).Close() calls

Fixes #32427

Change-Id: I4b863bd3836067dcc2eb3a9c3a7169656763d003
Reviewed-on: https://go-review.googlesource.com/c/go/+/180438


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