doc/go1.17: document crypto/rsa changes for Go 1.17

[gopherbot]

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

crypto/rsa

TODO: https://golang.org/cl/302230: fix salt length calculation with PSSSaltLengthAuto

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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 crypto/rsa changes for Go 1.17 #46006." in the body.

[dmitshur]

Hi @hkishn, are you able to help suggest an entry for the Go 1.17 release notes to resolve this TODO and document the change made in CL 302230?

You can either make a suggestion on this issue, or send a CL that edits doc/go1.17.html directly if that's easier. Thanks if you can help with this!

[FiloSottile]

This is a bug fix that resolves an error, not introduce a new one, so it's unlikely to break anyone, and very few applications need these key sizes, so I don't think we need to surface it in the release notes.