[Documentation] make some doc notes and warnings while validating optional values.

[royhsu]

// An optional value.
let message: String? = "hello"

// This will trigger the rules.
let result = try message.validated(by: rules)

// This will skip the rules due to optional chaining.
let result = try message?.validated(by: rules)

[royhsu]

Maybe we provide a better function name could improve this.

let result = try message.explicitlyValidated(by: rules)

[royhsu]

Maybe we provide a better function name could improve this.

let result = try message.explicitlyValidated(by: rules)

Accepted.