Better directive completions

[alcarney]

The language server can now offer completions for directives themselves. but it would be good to take that further to cover

• A directive's arguments (Refactor to be based on "features" #58)
• A directive's options (Provide completion suggestions for a directive's options #78)
• The actual directive's documentation (Initial completionItem/resolve support #284)

[alcarney]

The tricky part of providing documentation is that taking the docstring of the implementing class as the source is not practical simply because 90% of the time it does not contain user facing documentation. The only other approach I can think of is to copy the relevant docs into the codebase somewhere and do a look up, but that doesn't seem scalable....