UI test suite clarity changes: Rename tests/ui/SUMMARY.md and update rustc dev guide on error-pattern

[Oneirical]

To match convention, rename tests/ui/SUMMARY.md to tests/ui/README.md.

Also, remove misleading lines in the rustc development guide about error-pattern being "not recommended", when it really is just a last resort which should be used in the niche situations where it is useful.

r? @jieyouxu

[rustbot]

jieyouxu is currently at their maximum review capacity.
They may take a while to respond.

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

[jieyouxu]

Thanks, a wording nit, r=me otherwise.

[jieyouxu]

Nit: this might be worded a bit confusingly, maybe

For tests wishing to match against compiler diagnostics, error annotations should be preferred over //@ error-pattern, //@ error-pattern is imprecise and non-exhaustive.

[jieyouxu]

@bors delegate+ rollup

[bors]

✌️ @Oneirical, you can now approve this pull request!

If @jieyouxu told you to "r=me" after making some further change, please make that change, then do @bors r=@jieyouxu

[jieyouxu]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[bors]

☔ The latest upstream changes (presumably #144360) made this pull request unmergeable. Please resolve the merge conflicts.