More documentation issues

[jlbosse]

The docstring for curve_fit() does not mention that curve_fit() accepts kwargs and
that these get forwarded to LsqFit.levenberg_marquardt(). This made me miss the
options upper and lower (as described here first. The docs at https://julianlsolvers.github.io/LsqFit.jl/latest/ also don't mention upper or lower anywhere.

I am happy to create a PR to fix that, just tell me whether

1. You want me to update the docstring of curve_fit() to include kwargs and mention they get forwarded to levenberg_marquardt().
2. You want me to find out why the docstring for neither levenberg_marquardt() nor the struct LevenbergMarquardt show up. There is a docstring for LevenbergMarquardt in the docs, but it doesn't show up in the REPL.
3. You want me to add an example of the usage of upper and lower to the examples in the tutorial or API reference section of the docs.

[nluetts]

I had the exact same problem, would be great if the docs would be more explicit w.r.t. the kwargs.