[BUG][GRADLE] missing documentation for systemProperties and additionalProperties

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? not relevant
• Have you validated the input using an OpenAPI validator (example)? not relevant
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The documentation for the gradle plugin is very unclear. The sections additionalProperties and systemProperties are mentioned, but not further documented. There should be documentation which describes the config options for this sections.

openapi-generator version

Version 4.3.1

OpenAPI declaration file content or url

Not relevant for documentation issue.

Command line used for generation

Not relevant for documentation issue.

Steps to reproduce

Read documentation at https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator-gradle-plugin. Try to understand what is allowed in systemProperties or additionalProperties

Related issues/PRs

Suggest a fix

Link to documentation explaining systemProperties and additionalProperties like it is done for configOptions

This may also be true for other openapi plugins (especially maven), but I only looked into the gradle one deeper.

Read the Gradle plugin documentation under modules/openapi-generator-gradle-plugin and compare the existing configOptions section with the undocumented systemProperties and additionalProperties sections. Add explanations of the configuration options and link to the relevant documentation, including any supported details shown by the plugin documentation.