`merge_imports` documentation missing "Stable" info

[dogsoahc]

The section on the merge_imports option in Configurations.md is missing the line on whether it is stable or not.

(I realize that option is deprecated, but since other deprecated options still have such a line, I figured it should be there.)

Also, the section on newline_style has two empty lines preceding it, which is not causing any problems, but it's just bugging me because it's the only option with two and not one empty line before it.

[stefnotch]

The config options are defined here, and the boolean tells you whether they are stable (true) or not (false)

rustfmt/src/config/mod.rs

Line 32 in 0332da0

// `name: value type, is stable, description;`

merge_imports is an unstable option

rustfmt/src/config/mod.rs

Line 100 in 0332da0

merge_imports: MergeImports, false, "(deprecated: use imports_granularity instead)";

I figured that out with a bit of creativity. I went to the closed PRs and searched for "stabilize", because surely one of those options was stabilized in the past.

[ytmimi]

@dogsoahc if you or anyone else would be able to add the unstable tag to the docs that would be great

[dogsoahc]

@ytmimi I created a pull request with the change. I also added the tracking link to the style_edition option while I was at it. I hope I got it right.