One of the options of the new `banner-mode` property is `off`. YAML maps
`off` to `false` and since we are exposing the method from the
`SpringApplication` public class we can't change the signature to accept
a `String` and do the conversion ourselves.
This commit adds a dedicated section in the guide to warn users about
that particular situation. Adding quotes around the value prevents the
conversion.
Closes gh-4600
43b8e65eee