Document how to use LegacyCookieProcessor

Closes gh-6827

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

@@ -772,6 +772,38 @@ HTTPS connector:
 
 
 
+[[howto-use-tomcat-legacycookieprocessor]]
+=== Use Tomcat's LegacyCookieProcessor
+The embedded Tomcat used by Spring Boot does not support "Version 0" of the Cookie
+format out of the box, and you may see the following error:
+
+[indent=0]
+----
+	java.lang.IllegalArgumentException: An invalid character [32] was present in the Cookie value
+----
+
+If at all possible, you should consider updating your code to only store values
+compliant with later Cookie specifications. If, however, you're unable to change the
+way that cookies are written, you can instead configure Tomcat to use a
+`LegacyCookieProcessor`. To switch to the `LegacyCookieProcessor` use a
+`TomcatContextCustomizer` bean:
+
+[source,java,indent=0,subs="verbatim,quotes,attributes"]
+----
+	@Bean
+	public TomcatContextCustomizer customizer() {
+		return new TomcatContextCustomizer() {
+
+			void customize(Context context) {
+				context.setCookieProcessor(new LegacyCookieProcessor());
+			}
+
+		};
+	}
+}
+
+
+
 [[howto-use-jetty-instead-of-tomcat]]
 === Use Jetty instead of Tomcat
 The Spring Boot starters (`spring-boot-starter-web` in particular) use Tomcat as an