Improve message for not_unsafe_ptr_arg_deref lint

[stevenengler]

changelog: Improved message for the ['not_unsafe_ptr_arg_deref'] lint

Doesn't close any issue, but implements a suggestion from #3045 (comment).

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Manishearth (or someone else) soon.

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[Manishearth]

@bors r+

[bors]

📌 Commit 58491d3 has been approved by Manishearth

[bors]

⌛ Testing commit 58491d3 with merge 860cb8f...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 860cb8f to master...