cmd/go: document file language version semantics in go help buildconstraint

[findleyr]

Discussing with @matloob, we were surprised that https://go.dev/doc/toolchain was the best (only?) canonical documentation we could find for the fact that //go:build go1.22 is not just a build constraint, but also affects the language version within that file.

We should make this more apparent in documentation, by including this information in (at least) go help buildconstraint. It may also be worth mentioning this in go.dev/ref/mod, or other go command documentation.

[gabyhelp]

Similar Issues

• doc: mention language downgrading via `//go:build` #61894
• cmd/go: go directive is insufficiently documented for module authors to be able to make a decision about its value #30791
• cmd/go, cmd/compile: record language version to support language transitions #28221

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[findleyr]

Helpful @gabyhelp.

Not yet closing as a dupe of #61894 (though it's very closely related), since this issue is about a specific go command help page. But feel free to close this if you'd prefer.

[gopherbot]

Change https://go.dev/cl/595475 mentions this issue: cmd/go/internal/help: add documentation for language version downgrading