Update --pos tag behavior for single-value input

[KavishaHaswani]

Summary of the Pull Request

Updated the behavior of the --pos command-line argument in documentation.

References and Relevant Issues

Related to #4247

Detailed Description of the Pull Request / Additional comments

• A single value like --pos 500 now sets both x and y to 500.
• A trailing comma (e.g., --pos 500,) sets only x, using the default for y.

[learn-build-service-prod]

Learn Build status updates of commit 663248d:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/command-line-arguments.md	✅Succeeded	View

For more details, please refer to the build report.

[learn-build-service-prod]

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[KavishaHaswani]

Note: This docs PR is related to the main code PR (microsoft/terminal#19409). The main code changes are pending review, so this PR depends on that one.