More failsafe demo maker

[sveetch]

The lotus_demo command is making a lot of computation with articles and so require a minimum article length, however this is not enforced by validation or documentation.

Let's add some argument validation and improve command doc helps.

• Minimal article length is around 7;
• Add some mention about the factory usage which is not totally safe and can lead sometime to failure on unique constraints (like sometime the Author factory can try to use the same user name twice);