Add a section for assertions and note marked output

[philderbeast]

A documentation change only, add a section for assertions in the haddocks for cabal-testsuite and explain that we can only make assertions about marked output.

As rendered:

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• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[philderbeast]

With the module re-exporting itself, does anyone know how to avoid the duplication seen in the rendered docs of some functions like assertOutputContains?

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-07-05 09:36 UTC · Rule: default · triggered by rule Put pull requests in the rebase+merge queue
• ✅ Checks passed · on draft merge queue: checking master (bc15653) and [#12077 + #12073] together #12089
• ✅ Merged — 2026-07-05 12:13 UTC · at b0170f8ccc0dcd6646d69cf4dd001e3b2754996c · merge

This pull request spent 2 hours 36 minutes 59 seconds in the queue, including 1 hour 37 minutes 38 seconds running CI.

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  • Add a section for assertions and note marked output #12073
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