QDoc includes a showinternal command line option and configuration variable designed to generate documentation for code marked with \internal commands. This feature worked in earlier versions but has degraded over time and is now non-functional, despite recent build system fixes (QTBUG-118176) that correctly pass the option to QDoc.

With the recent implementation of private member documentation support, restoring showinternal functionality becomes even more valuable for generating comprehensive internal documentation that includes both private implementation details and internal APIs.

Acceptance Criteria

• showinternal command line option functions correctly
• showinternal configuration variable works in .qdocconf files
• Internal documentation (marked with \internal) appears in generated output when enabled
• Feature works across all output formats (HTML, DocBook, WebXML)
• Backward compatibility maintained - no documentation changes when showinternal is disabled
• Test coverage includes validation of internal documentation generation