add LazyLock to "Higher-level sync" docs

[pitaj]

Partially addresses #125615

There's room for improvement but I figured it would be good to get something in there sooner rather than later.

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[jhpratt]

While the docs current state "global variable" and that is probably the most common use case, it is very much permitted to use these with locals. r=me when changed.

[workingjubilee]

I had worked out a PR (#125696) to fully address that issue and another, and my wording for this specific part is in 66ee452

Feel free to reappropriate it in part, and I'm happy to drop that commit if this lands.

[bors]

☔ The latest upstream changes (presumably #125989) made this pull request unmergeable. Please resolve the merge conflicts.