Meta issue: Documentation improvements

[jvoigtlaender]

Here is a list of open PRs that are "only" about improving the documentation at various places. We do want good and helpful documentation. These PRs don't change anything in terms of the behavior of functions in core. They should be cheap to merge:

• https://github.com/elm-lang/core/pull/341
• https://github.com/elm-lang/core/pull/393
• https://github.com/elm-lang/core/pull/406
• https://github.com/elm-lang/core/pull/436
• https://github.com/elm-lang/core/pull/450

Can we have those merged before any new release of core? The best way to ensure that is to merge them right now, of course. I can do the merging (and resolution of conflicts if there are any), if you give green light for that, @evancz.