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

[gopherbot]

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

reflect

TODO: https://golang.org/cl/266197: add Method.IsExported and StructField.IsExported methods

TODO: https://golang.org/cl/281233: add VisibleFields function

TODO: https://golang.org/cl/284136: panic if ArrayOf is called with negative length

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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 reflect changes for Go 1.17 #46019." in the body.

[gopherbot]

Change https://golang.org/cl/318011 mentions this issue: doc/go1.17: fill in TODO for reflect package

[gopherbot]

Change https://golang.org/cl/323069 mentions this issue: doc/go1.17: document reflect changes