doc/go1.17: document os changes for Go 1.17

[gopherbot]

As of filing this issue, the Go 1.17 draft release notes contained the following HTML:

os

TODO: https://golang.org/cl/268020: avoid allocation in File.WriteString

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This TODO needs to be resolved by July 1 so that we can have complete release notes for Beta 1.

A sequence of steps to resolve this issue may be:

1. Come up with a draft release note entry in this issue, then move the issue to NeedsFix state.
2. Send a CL with "doc/go1.17:" prefix and include "For doc: write Go 1.17 release notes #44513. Fixes doc/go1.17: document os changes for Go 1.17 #46018." in the body.

[gopherbot]

Change https://golang.org/cl/317910 mentions this issue: doc/go1.17: Add release notes entry for os.