Move too_long_first_doc_paragraph to nursery

[xFrednet]

See: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/too_long_first_doc_paragraph.20to.20nursery.3F/near/476448239

changelog: none

I'll update the changelog as well.

[rustbot]

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[xFrednet]

I'll "Roses are approved" this directly according to Zulip

---

Roses red approved,
Violets are moved,
Lints are allowed,
Fixes will be made

[bors]

📌 Commit fcf4646 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit fcf4646 with merge 7ce1eea...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 7ce1eea to master...

[flip1995]

This is already in stable, beta and master branch in the Rust repo. I also remember to do a last minute backport last release to stable and beta in the Rust repo. So this was already done before and I think you even included it in the changelog. So ✔️