List of chesster commands out-of-date

[hopffgam]

Users can get a list of chesster commands by sending the message "help" to chesster. However, that list misses at least a description of the command to set yourself (un)available. Also it seems some commands (like "starter guide") are not working anymore. Goal of this issue is to investigate and bring the command list up to date, not necessarily to identify and fix broken commands (which could be done in separate individual issues).