docs(streaming): document redactedContent event type

[awsarron]

Description

document redactedContent event type introduced in strands-agents/sdk-python#99

Type of Change

• Content update/revision

Motivation and Context

New SDK feature.

Areas Affected

• Streaming / Async Iterators
• Streaming / Callback Handlers

Screenshots

N/A

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

N/A

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.