Add a note explaining that double backslashes are unnecessary in YAML

Closes gh-1989

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -496,6 +496,10 @@ to `application.properties`, e.g.
 	server.tomcat.internal_proxies=192\\.168\\.\\d{1,3}\\.\\d{1,3}
 ----
 
+NOTE: The double backslashes are only required when you're using a properties file for
+configuration. If you are using YAML, single backslashes are sufficient and a value
+that's equivalent to the one shown above would be `192\.168\.\d{1,3}\.\d{1,3}`.
+
 Alternatively, you can take complete control of the configuration of the `RemoteIpValve`
 by configuring and adding it in a `TomcatEmbeddedServletContainerFactory` bean.