Extend description of GOAMD=v4

[bboreham]

This code is gated on GOAMD=v4, so we should include that case in the wiki.
https://github.com/golang/go/blob/608acff8479640b00c85371d91280b64f5ec9594/src/cmd/compile/internal/ssagen/intrinsics.go#L1339-L1340

The proposed text is my understanding based on a Slack conversation with Dominik Honnef.

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Bryan Boreham:

Patch Set 1:

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Sean Liao:

Patch Set 1:

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