doc/go1.17: document changes to os package
doc/go1.17: document changes to os package

Documents the changes to File.WriteString method.

For #44513.
Fixes #46018.

Change-Id: I3a8ef9df9f84662614d54802710bd705d626b995
Reviewed-on: https://go-review.googlesource.com/c/go/+/317910
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go