Preface 'cares' with 'only'

[mehcode]

Minor doc edit to make it clear that collect only needs the collection type and is not just being caring.

[rust-highfive]

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

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[QuietMisdreavus]

@bors r+ rollup

Thanks!

[bors]

📌 Commit bbdff02 has been approved by QuietMisdreavus

[bors]

⌛ Testing commit bbdff02 with merge cbbe17a...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: QuietMisdreavus
Pushing cbbe17a to master...