build/constraint: update doc to mention a feature added in Go 1.17

[cristiangreco]

The pkg documentation mentions that the "//go:build" syntax "will be"
added in Go 1.17. In fact, it has been added in that Go release, so the
documentation can now be updated.

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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This PR (HEAD: 4371886) has been imported to Gerrit for code review.

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Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 2: Run-TryBot+1 Auto-Submit+1 Code-Review+2

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Patch Set 2:

(1 comment)

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Message from Gopher Robot:

Patch Set 2: TryBot-Result+1

(1 comment)

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This PR is being closed because golang.org/cl/415774 has been merged.