[Bug or feature request] - document `--language-version` in `dart format --help`

[cameron1024]

I'm writing a tool that pipes generated Dart code into/out of dart format via
stdio, and was looking for a way to set the language version for the formatter
via the CLI (since there is no language constraint in the pubspec.yaml -
there is no pubspec.yaml!).

I came across this issue: #1402 ,
which suggests there is a --language-version flag, but after following the
discussion, it's not super clear what the status of this flag is. I can pass
the flag and it works, but I'm a bit worried it's been intentionally left
undocumented as a hint that it's experimental/unstable/subject to change, at
which point I wouldn't rely on it for a core part of a tool that's going to
process all the Dart code in my organization 😅 .

For context, when I run dart format --help I get this:

Idiomatically format Dart source code.

Usage: dart format [options...] <files or directories...>
-h, --help                     Print this usage information.
-v, --verbose                  Show all options and flags with --help.
-o, --output                   Set where to write formatted output.

          [write] (default)    Overwrite formatted files on disk.
          [show]               Print code to terminal.
          [json]               Print code and selection as JSON.
          [none]               Discard output.

    --set-exit-if-changed      Return exit code 1 if there are any formatting changes.

Run "dart help" to see global options.

on Dart 3.8.1.

If this is intentional, feel free to close this issue. I was just having
trouble finding any documentation on this feature. Seeing it --help would be
great for discoverability.

Thanks 😁