Wordsmith the edition documentation a bit more

Wordsmith the edition documentation a bit more

This hopes to clear up a few recent PRs about the `edition` key in the
manifest. It shuffles things around so the defaults are talked about in
two different paragraphs. The first one talks about how `cargo new` uses
the 2018 edition and is the only paragraph to use the word "default".
The second section talks about the usage of 2015 with no `edition` field
present.

This didn't end up being quite as clear as I hoped it might be, but I'm
hoping that this provides a bit more emphasis or otherwise just
restructures things a bit to be more clear.