Skip XML comment langword diagnostics for attributed code tags

[meziantou]

Summary

• Ignore <c> and <code> XML comment elements when they already carry attributes, since they are not plain keyword markup.
• Update the MA0154 documentation to clarify that the rule targets plain <c> and <code> usage.
• Add a regression test covering attributed <c> content.

Testing

• Added analyzer test coverage for an attributed XML comment example.
• Documentation and analyzer changes were validated by the existing test suite expectations in the affected rule area.