Improve startup error message

This commit improves the startup error message so that it does not
reference  `--debug` anymore. Such command-line switch only works when
the application is started using `java -jar`.

The error message now refers directly to a section of the documentation
that provides more details and links to more useful examples.

Closes gh-6593

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

@@ -46,6 +46,11 @@ When your application starts you should see something similar to the following:
 By default `INFO` logging messages will be shown, including some relevant startup details
 such as the user that launched the application.
 
+
+
+[boot-features-startup-failure]
+=== Startup failure
+
 If your application fails to start, registered `FailureAnalyzers` get a chance to provide
 a dedicated error message and a concrete action to fix the problem. For instance if you
 start a web application on port `8080` and that port is already in use, you should see
@@ -69,6 +74,20 @@ something similar to the following:
 NOTE: Spring Boot provides numerous `FailureAnalyzer` implementations and you can
 <<howto.adoc#howto-failure-analyzer,add your own>> very easily.
 
+If no failure analyzers are able to handle the exception, you can still display the full
+auto-configuration report to better understand what went wrong. To do so you need to
+<<#boot-features-external-config,enable the `debug` property>> or
+<<#boot-features-custom-log-levels,enable `DEBUG` logging>> for
+`org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer`.
+
+For instance, if you are running your application using `java -jar` you can enable the
+`debug` property as follows:
+
+[indent=0,subs="attributes"]
+----
+	$ java -jar myproject-0.0.1-SNAPSHOT.jar --debug
+----
+
 
 
 [[boot-features-banner]]