chore(config): add log.max_depth example config

etc/emqx.conf

@@ -439,6 +439,13 @@ log.file = emqx.log
 ## Default: No Limit
 #log.chars_limit = 8192
 
+## Maximum depth for Erlang term log formatting
+## and Erlang process message queue inspection.
+##
+## Value: Integer or 'unlimited' (without quotes)
+## Default: 20
+#log.max_depth = 20
+
 ## Log formatter
 ## Value: text | json
 #log.formatter = text