Document PathRequest.toH2Console and using the console with Spring Security

[wilkinsona]

See #3734 for background. We added PathRequest.toH2Console() in 2.0 but it doesn't appear to be mentioned in the reference documentation. I think it'd be good to mention it in https://docs.spring.io/spring-boot/docs/2.5.x/reference/htmlsingle/#features.sql.h2-web-console.

[wilkinsona]

Closing in favor of #29932.