Proposal: Surface clearer help documentation around the default subcommand

Commands with default subcommands do not have discoverable usage information without digging into the subcommand itself. Below, I'll outline the structure of a simple project as well as two proposals for updating the help information for commands that have a default subcommand.

Simply put, a command with a default subcommand likely doesn't have any information it runs itself and, as such, should specify the information that is available by running the default subcommand.

## Structure

For ease of clarity, repeating the structure of my project here:

Command: `zinc`
Subcommands: `lint`, `sync`
Default Subcommand: `sync`

## Proposal 1 - Highlighting the Default Subcommand

### Problem Statement

The output of `zinc --help` is such that the default subcommand is not made clear.

This is what prints:

```
SUBCOMMANDS:
lint Performs basic linting against a Zincfile to identify issues and errors.
sync Syncs local files with remote files as defined by a Zincfile.
```

### Proposed Solution

I'm proposing we highlight the default subcommand somehow:

```
SUBCOMMANDS:
lint Performs basic linting against a Zincfile to identify issues and errors.
sync (default) Syncs local files with remote files as defined by a Zincfile.
```

## Proposal 2 - Prefer Default Subcommand Usage

### Problem Statement

If I specify a default subcommand in my project, while I would allow someone to run `zinc sync `, specifying a default subcommand likely implies that I'm happy with `zinc `. As such, I think that the output of ` --help` should be refined.

Right now, here's what prints out when I run `zinc --help`:

```
OVERVIEW: Zinc is a command-line tool for keeping local files in sync with files hosted outside of your folder or
repository.

USAGE: zinc

OPTIONS:
-h, --help Show help information.

SUBCOMMANDS:
lint Performs basic linting against a Zincfile to identify issues and errors.
sync Syncs local files with remote files as defined by a Zincfile.
```

Here is the output of `zinc sync --help`:

```
OVERVIEW: Syncs local files with remote files as defined by a Zincfile.

USAGE: zinc sync [--file ] [--verbose]

OPTIONS:
-f, --file The Zincfile to parse and use for syncing. (default: Zincfile)
--verbose Logs additional debug messages if enabled.
-h, --help Show help information.
```

### Proposed Solution

I'm not sure what the right answer is here, but I imagine we should see an example of the default command being run, along with additional subcommands still.

I'm not sold on this, but here's an example:

```
OVERVIEW: Zinc is a command-line tool for keeping local files in sync with files hosted outside of your folder or
repository.

USAGE:
zinc --file
zinc

OPTIONS:
-f, --file The Zincfile to parse and use for syncing. (default: Zincfile)
--verbose Logs additional debug messages if enabled.
-h, --help Show help information.

SUBCOMMANDS:
lint Performs basic linting against a Zincfile to identify issues and errors.
sync (default) Syncs local files with remote files as defined by a Zincfile.
```

## Additional Notes

I'd like some additional feedback on the proposed solutions. I'm poking around at a variety of commands that I use day-to-day in my Swift projects as well as some outside of the Swift ecosystem and there's a variety of answers, but almost all at least provide some help documentation for a default subcommand within the help documentation of the command itself if requested.

Start by reviewing the issue's `zinc --help` and `zinc sync --help` examples and the two proposed approaches. Determine which default-subcommand behavior the project wants, then verify that help output exposes the default command's usage and still documents other subcommands.

Written by the indexing model from the issue text.