Document that -config-dump shows sensitive info

Why:
Avoid surprising users that may not expect that passwords might be
printed in the config dump output

What:
Update manpage

miniflux.1

@@ -25,7 +25,7 @@ Load configuration file\&.
 .PP
 .B \-config-dump
 .RS 4
-Print parsed configuration values\&.
+Print parsed configuration values. This will include sensitive information like passwords\&.
 .RE
 .PP
 .B \-create-admin