--include / --files documentation is confusing

When I run `dotnet format --files .\src\ProjectFolder\Program.cs -v detailed` I always see

```
Formatting code files in workspace 'SOLUTION_FOLDER\Solution.sln'.
Warnings were encountered while loading the workspace. Set the verbosity option to the 'diagnostic' level to log warnings.
Formatted 0 of 321 files.
Format complete in 5402ms.
```

I would have expected to see only one file formatted.

it works, when I run `dotnet format --folder . --files .\src\ProjectFolder\Program.cs -v detailed` (kudos to https://github.com/dotnet/format/issues/403)

But in the README it's described as `dotnet format --include Programs.cs Utility\Logging.cs` without further explanation (https://github.com/dotnet/format#how-to-use) - maybe it could be improved?

**Background:**

The reason I want to specify only specific files, is to run it as a pre-commit hook as quickly as possible.