['arc_with_non_send_sync`] documentation edits

[talagrand]

Arc's documentation uses the term "thread"; aligning to that terminology. Fix casing of "Rc".

changelog: None

[rustbot]

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[talagrand]

Also, unrelated to this doc update, but kudos for this lint. It found a number of soundness issues in our codebase when we upgraded (crossing an ABI boundary meant we didn't get Rust's usual typesystem-level enforcement on smuggling non-Send types across threads).

[Jarcho]

Thank you. @bors r+

[bors]

📌 Commit c2d071c has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit c2d071c with merge 93ee420...

[bors]

💔 Test failed - checks-action_test

[Jarcho]

@bors retry

[bors]

⌛ Testing commit c2d071c with merge 37b8ae7...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 37b8ae7 to master...