Mention TLS-terminating proxy as a usecase for h2c

Closes gh-25855

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/docs/asciidoc/howto/webserver.adoc

@@ -213,6 +213,7 @@ You can enable HTTP/2 support in your Spring Boot application with the configpro
 Both `h2` (HTTP/2 over TLS) and `h2c` (HTTP/2 over TCP) are supported.
 To use `h2`, SSL must also be enabled.
 When SSL is not enabled, `h2c` will be used.
+You may, for example, want to use `h2c` when your application is <<howto.webserver.use-behind-a-proxy-server,running behind a proxy server>> that is performing TLS termination.
 The details of the `h2` support depend on the chosen web server and the application environment, since that protocol is not supported out-of-the-box by all JDK 8 releases.