Document plugging in custom sanitisation rules with a SanitizingFunction bean

https://github.com/spring-projects/spring-boot/issues/27840 added support for pluggable sanitisation by defining a `SanitizingFunction` bean. It's [described in the release notes](https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.6-Release-Notes#pluggable-sanitization-rules). We should add something to the reference documentation too.