Add note to type mismatch errors related to specialization.

[rphmeier]

Specifically, default associated types cannot be projected as specific types.

Fixes#33481 (which was actually by design)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Aatch (or someone else) soon.

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[rphmeier]

It could also be beneficial to add a separate error code for this issue; I have a strategy in mind for doing so if it proves to be better.

[rphmeier]

cc @aturon

[bors]

☔ The latest upstream changes (presumably #33425) made this pull request unmergeable. Please resolve the merge conflicts.

[rphmeier]

Closing in favor of new PR.