Add CLI documentation for #3243

We added new --debug option in #3243, adding it to CLI flags

Fixes #4199

docs/asciidoc/static/command-line-flags.asciidoc

@@ -38,7 +38,10 @@ Logstash has the following flags. You can use the `--help` flag to display this
 
 -t, --configtest
   Checks configuration and then exit. Note that grok patterns are not checked for 
-  correctness with this flag
+  correctness with this flag. 
+  Logstash can read multiple config files from a directory. If you combine this 
+  flag with `--debug`, Logstash will log the combined config file, annotating the
+  individual config blocks with the source file it came from.
 
 -h, --help
   Print help