Spring-boot: Document adding multiple connectors for Jetty and Undertow

[mbhave]

It's possible to add additional connectors on TomcatServletWebServerFactory. We should make that consistent unless there was a specific reason for not doing so on the reactive side.

Related #528.

[mbhave]

For Jetty and Undertow this can be done with a JettyServerCustomizer and UndertowBuilderCustomizer respectively. We can use this issue to add support for Tomcat and document how to configure multiple connectors for all three.

[mbhave]

The experience with Tomcat vs Jetty/Undertow is slightly inconsistent. With Tomcat you need to use the addAdditionalConnectors method. I don't think there's a way to add a connector without creating a new WebServerFactory. With Jetty and Undertow it's possible to do so via customizers.

We should try and make them all consistent.

[wilkinsona]

I don't think there's a way to add a connector without creating a new WebServerFactory.

It still wouldn't quite be consistent, but can't you use a WebServerFactoryCustomizer to avoid needing to create the factory?

[mbhave]

oh yeah, you're right. Wonder if we should change this section of the doc to use the customizer then?