doc: clarify os.(*File).Close() behaviour

[icholy]

The current documentation for os.(*File).Close() doesn't specify whether it's safe to call Close multiple times.

This is useful for code like this:

func doSomething() error {
    f, err := os.Create("foo")
    if err != nil {
        return err
    }
    defer f.Close()
    if _, err := f.Write([]byte("bar"); err != nil {
        return err
    }
    return f.Close()
}

System details

go version go1.12 linux/amd64
GOARCH="amd64"
GOBIN="/home/icholy/go/bin"
GOCACHE="/home/icholy/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/icholy/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.12 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.12
uname -sr: Linux 4.4.0-148-generic
LSB Version:	core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch:core-4.1-amd64:core-4.1-noarch
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.6 LTS
Release:	16.04
Codename:	xenial
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.23-0ubuntu11) stable release version 2.23, by Roland McGrath et al.
gdb --version: GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1

[gopherbot]

Change https://golang.org/cl/180438 mentions this issue: doc: clarify safety of mupltiple and concurent os.(*File).Close() calls