doc/go1.17: document net/http/httptest changes for Go 1.17

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

net/http/httptest

TODO: https://golang.org/cl/308950: panic on non-3 digit (XXX) status code in Recorder.WriteHeader

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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 net/http/httptest changes for Go 1.17 #46016." in the body.

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Change https://golang.org/cl/319190 mentions this issue: doc/go1.17: document fix to ResponseRecorder.WriteHeader

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Change https://golang.org/cl/322891 mentions this issue: doc/go1.17: document net/... changes