NIFI-11157 - Doc update for scheduling strategy for MergeContent/Record

[pvillard31]

Summary

NIFI-11157
Should also go into support/nifi-1.x

Tracking

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Issue Tracking

• Apache NiFi Jira issue created

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• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
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  • JDK 11
  • JDK 17

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Documentation

• Documentation formatting appears as expected in rendered files

[turcsanyip]

Thanks @pvillard31!

+1 LGTM
Merging to main and 1.x