`Gen::new` and `Gen::from_size_and_seed` have confusing documentation

https://github.com/BurntSushi/quickcheck/blob/692527d1d9fc9043a70b7d2bf436902e2d0f595b/src/arbitrary.rs#L45-L46

I don't understand what

> but is and should not be used

is supposed to say. Perhaps "is *not*"?

Open src/arbitrary.rs at lines 45-46 and read the documentation for Gen::new and Gen::from_size_and_seed in context. Clarify the confusing wording, then verify that the rendered Rust documentation accurately describes both constructors.

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