Mention security consideration in ForwardedHeaderFilter javadoc

See https://github.com/spring-projects/spring-framework/pull/23582#issuecomment-725911525

We should elaborate about security considerations about Forwarded headers and proxies in the filter's Javadoc, just like in ["Forwarded" section in the reference documentation](https://docs.spring.io/spring-framework/docs/current/reference/html/web.html#filters-forwarded-headers).