Clarify usage of sub-sub commands groups

Since %0.11.0 release, commands are grouped into groups, and makes them non as easy to discover as it used to be when all commands could be found at the root.

Silkaj has groups of commands, you can find them as such:

silkaj blockchain -h
silkaj money -h
silkaj money transfer -h
silkaj wot revocation -h

• https://forum.duniter.org/t/silkaj-v0-11-0-release/10087#new-cli-design-3
• https://forum.duniter.org/t/questions-silkaj-v0-11-0-nouvelle-cli-transfer/10619/2?u=moul