Mention in doc comment that len field of scrypt::Params is used only in PasswordHasher

[sorairolake]

I know this has already been mentioned in the comment.

password-hashes/scrypt/src/params.rs

Lines 14 to 15 in 7ff9080

	#[allow(dead_code)] // this field is used only with the `PasswordHasher` impl
	pub(crate) len: usize,

However, doc comments of Params::new and Params::RECOMMENDED_LEN does not mention this. So, I suggest mentioning this in doc comments as well. I think this makes it easier to understand that len parameter of Params::new is used only in PasswordHasher and any value is meaningless when PasswordHasher is not used.

[tarcieri]

#415 expressed some similar concerns/confusion