Show hint how to find out what data can be printed

This hint is unnecessary for inspect commands since this is the default. But for other commands, I regularly find out again this trick.

config/formatting.md

@@ -87,3 +87,10 @@ docker inspect --format "{{upper .Name}}" container
 docker inspect --format='{{range .NetworkSettings.Networks}}{{println .IPAddress}}{{end}}' container
 ```
 {% endraw %}
+
+# Hint
+
+To find out what data can be printed, show all content as json:
+```
+docker container ls --format='{{json .}}'
+```