[Feature Request] help command in watchman sub command

* Purpose
List the usage and description for watchman sub commands
* Why
It is not that good to split out an error like `wrong number of arguments to 'log'`, it will be better if we list the use case of that command, and that's how we can help the user input the right command and know what is the command doing.
* PS
I can do this PR if the feature request has been review and pass. Thanks!