Improve documentation of SinkSH

Summary:

• Add support for positional arguments, options, flags in the syntax tree
• Add automatic generation of usage string

TODO:

• Better document the --reduce option in the list command
• Get the parent command for sub-commands so we could show help for trace on and not just on
• Pass the syntax to the implementation of the command so commands can easily show help on wrong usage. Also, SyntaxTree could do automatic input validation in the future, this could also help.
• Even with the new documentation, some command could still be confusing. Should we add some usage examples?

Reviewers: cmollekopf

Reviewed By: cmollekopf

Tags: Sink

Differential Revision: https://phabricator.kde.org/D14547