Enhancement: Update command line help message for --platform

[weisunOW]

Description

Address #44

• Update the command line help message to mention that maccatalyst platform is not included when --platform argument is omitted.

Detailed Design

N/A

Documentation Plan

How has the new feature been documented? Have the relevant portions of the guide been updated in addition to symbol-level documentation?

N/A

Test Plan

How is the new feature tested?

N/A

Source Impact

What is the impact of this change on existing users? Does it deprecate or remove any existing API?

This PR adds additional information to the help message of command option --platform to explain the actual behaviour when the option is omitted.

Actual behaviour:

Line 182 - Sources/CreateXCFramework/PackageInfo.swift

func supportedPlatforms () throws -> [TargetPlatform] {

        // if they have specified platforms all good, if not go everything except catalyst
        let supported = self.options.platform.nonEmpty ?? TargetPlatform.allCases.filter { $0 != .maccatalyst }
        
        // Omitted ...
}

Checklist

• [ ] I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[weisunOW]

Thanks @bok-