docs: clarify aria snapshot matching - #42596
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While investigating #38833, I also exercised failing ARIA snapshot assertions with coding agents. Better trace diffs would still help people review a failure, but these agents often need to repair the assertion from the terminal output. The recurring mistakes were treating templates as complete trees, misunderstanding
contain,equal, anddeep-equal, and copying invalid YAML or incorrectly escaped strings.Summary
Background
Some nested YAML examples omit the mapping colon required for child entries, and the C# verbatim-string example escapes the regular expression incorrectly. The matching and snapshot-update sections also describe behavior differently from the current parser, matcher, and test-runner update modes. These corrections make copied examples work and document the behavior users see at runtime.