Support @PageImage in in-source documentation comments

[AryanBagade]

Bug/issue #, if applicable: #1403

Summary

Previously, @PageImage directives were rejected when used in documentation comments with a warning suggesting to use a documentation extension file instead. This change removes that restriction, allowing developers to specify page images directly in their source code comments.

This aligns @PageImage behavior with @Available, which is already supported in documentation comments.

Implementation:

• Removed validation that rejected @PageImage in Metadata.validateForUseInDocumentationComment()
• Removed clearing of pageImages array for documentation comments

Dependencies

None.

Testing

1. Build the project with swift build
2. Run tests with swift test --filter SymbolTests
3. Verify the new tests pass:
   • testParsesPageImageDirectiveFromDocComment
   • testParsesMultiplePageImageDirectivesFromDocComment

Checklist

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary (N/A - no documentation changes needed)

[heckj]

@swift-ci please test