Argument Clinic: add reference for all directives

[erlend-aasland]

We need a good reference for all Argument Clinic directives (@getter, output, etc.). Some of the directives accept one or more parameters; perhaps we can use typical CLI documentation for inspiration (@critical_section [arg1] [arg2]).

[Lincoln-developer]

Hey @erlend-aasland , I would love to work on this issue if its not taken yet to be worked on. Thanks!

[erlend-aasland]

Hey @erlend-aasland , I would love to work on this issue if its not taken yet to be worked on. Thanks!

Great; feel free to propose a PR.