Recommend that data should be no longer than REASONABLE_SECRET_DATA_SIZE

[dfed]

See this file for the constant. Follow up to #246 and #247.

Given that we have the known upper desired limit of keychain secret size (thank you @KhaosT!), I think it's reasonable for us to create a new KeychainError case valueTooLarge, and check that the entered data isn't above that limit before interacting with the securityd. If and when we do that, we'd want to change this /// - Important to a /// - Precondition.

Open to someone putting out a PR that does that (or I can get to it at a future date). We'd want to do this check within Keychain's setObject(_:forKey:options:).

But for now, let's make sure our documentation is telling our customers the right thing per #246 (comment).

[NickEntin]

supernit: It looks like it supports up to 4kb, rather than requiring it be smaller than that.

Suggested change

	/// - Important: Inserted data should be smaller than 4kb.
	/// - Important: Inserted data should be no larger than 4kb.

[NickEntin]

create a new KeychainError case keyTooLarge

I think the REASONABLE_SECRET_DATA_SIZE is referring to the value, not the key. Or maybe both? Might be worth adding both keyTooLarge and valueTooLarge errors?

[dfed]

@NickEntin 🤦‍♂️ I def meant valueTooLarge not keyTooLarge. Good catch. I've updated the description.

Regarding key size: I haven't found any explicit key-size comparisons in the linked code. We can cross that bridge if we find it.