docs: Fix too_long_first_doc_paragraph: line -> paragraph

[evanj]

The documentation for too_long_first_doc_paragraph incorrectly says "line" where it should say "paragraph".

Fix a minor typo: doscstring -> docstring.

Also do a few tiny edits to attempt to make the wording slightly shorter and clearer.

changelog: [too_long_first_doc_paragraph]: Edit documentation

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @xFrednet (or someone else) some time within the next two weeks.

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[xFrednet]

LGTM, thanks!

---

Roses are red,
this paragraph short,
it approves the PR,
for it to be merged

[bors]

📌 Commit 0bcc6f8 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0bcc6f8 with merge 9cf416d...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 9cf416d to master...