Correct the docs on str::trim_matches

[withoutboats]

This pattern cannot be a str because str's pattern is not double-ended.

[rust-highfive]

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[withoutboats]

r? @steveklabnik

[birkenfeld]

While at it, could we add &[char] to the list of acceptable types for the trim methods?

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit ae14223 has been approved by steveklabnik