docs(lambda): explain event filter criteria composition

[caglarkc]

Issue # (if applicable)

Closes #23214.

Reason for this change

The originally malformed FilterCriteria example was fixed in #23085, but this issue remains open for a general explanation of how to construct and combine filters.

Description of changes

Explain FilterCriteria.filter serialization, AND across fields, OR across values and filter entries, and source-specific data fields. Add a DynamoDB event pattern with event-name alternatives and a nested string-prefix comparison, together with its exact FilterCriteria JSON.

Describe any new or updated permissions being added

None. This is a documentation-only change.

Description of how you validated changes

• Built aws-cdk-lib using the dependency versions pinned by the current main branch.
• Compiled and translated the new README example with jsii-rosetta using the repository fixture.
• Executed the exact README example against a DynamoDB stream and Lambda event source mapping. Compared its synthesized FilterCriteria to the JSON in the documentation and checked the parsed event pattern.
• Ran jest aws-lambda/test/event-source-mapping.test.ts --runInBand --coverage=false (47 tests passed).
• Reviewed the documentation against the implementation and checked git diff --check.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

📌 Commit eac16ff

This PR adds a "Defining event filter criteria" subsection to the aws-lambda README (+50/-0), explaining how to compose event source mapping filter criteria: wrapping patterns in FilterCriteria.filter(), using FilterRule helpers, and the AND/OR combination semantics, with a worked DynamoDB example and its synthesized JSON. The documented serialization behavior, the FilterRule.or and FilterRule.beginsWith/prefix output, and the combination semantics all match the implementation and the linked AWS event-filtering guidance; the example resolves against the module's default fixture and demonstrates the feature. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

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