Document global options + improve option names

[avehtari]

Some of the option names are ugly. As they are not yet publicly documented, I think we should change these

• projpred.prll_cv -> projpred.parallel_cv

These are documented, but still would be better to change

• projpred.warn_prj_drawwise -> projpred.warn_proj_drawwise
• projpred.check_conv -> projpred.check_convergence
• projpred.prll_prj_trigger -> projpred.parallel_proj_trigger

There are undocumented global options that should be documented

[fweber144]

Thanks. I'm open to renaming these options. Concerning projpred.prll_cv, I just want to add that it is (indirectly) documented because it appears in the default of argument parallel of cv_varsel.refmodel().

For the yet-undocumented global options that should be documented, I'm also open to documenting these. Just for the background: I did not document all global options because some referred to yet-experimental features for which implementation (including theoretical investigations, documentation, and tests) is not finished yet (see, e.g., global option projpred.glm_fitter) or they were added only for internal purposes (see, e.g., many of the projpred.warn_[...] options, like projpred.warn_additive_experimental, which were added only to deactivate the corresponding warning messages in the unit tests).