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

[shepmaster]

quickcheck/src/arbitrary.rs

Lines 45 to 46 in 692527d

	/// vector, but is and should not be used to control the range of a
	/// randomly generated number. (Unless that number is used to control the

I don't understand what

but is and should not be used

is supposed to say. Perhaps "is not"?