Add note to type mismatch errors related to specialization.

[rphmeier]

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

This is the same as #33492, but rebased onto the latest master and in a different local branch.

Fixes#33481

[rust-highfive]

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

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[bors]

☔ The latest upstream changes (presumably #33137) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

@rphmeier oh dear sorry for the delay! This has been inactive for awhile now, but could you resubmit with a rebase? Thanks!