add a comment to lib/src/display.dart

[devoncarew]

• add a comment to lib/src/display.dart

Adding this comment to provide context on the lib/src/display.dart file. This is especially relevant as this file will need to be updated as part of a migration to analyzer v8.0.0 / v9.0.0, which involves several element model changes.

I'd taken a stab at doing the upgrade (w/ @scheglov 's help), but saw a few test failures and things like type aliases not being documented. The upgrade itself would likely only take a day or two; hopefully this comment provides some context.

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