Update documentation

[kieranrcampbell]

As it stands the documentation for the main function is pretty poor. In particular -

• The title is "Fit a Ouija object." - could be more informative
• The description is totally non-informative
• The "return" section is close to useless
• The description should contain a section on how to control inference - this can be taken from the main vignette
• The description should at least reference the downstream analysis functions or reference to the relevant vignette section
• The arguments sent to rstan should be documented and defaults provided!

[kieranrcampbell]

Addressed in latest commit (apart from references to vignette)