[FLINK-14123][DOCS]Update release notes for flink-1.9

[liupc]

What is the purpose of the change

Update the release notes docs for flink-1.9 on the memory management. We suggest users to manually lower the taskmanager.memory.fraction conf value to avoid OOMs in some case.

Brief change log

• Update release notes for flink-1.9 on the memory management.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

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Automated Checks

Last check on commit 4fae019 (Mon Oct 21 09:37:37 UTC 2019)

Warnings:

• This pull request references an unassigned Jira ticket. According to the code contribution guide, tickets need to be assigned before starting with the implementation work.

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[tillrohrmann]

Thanks for this documentation improvement @liupc. LGTM. Merging now.