[FLINK-15684][docs] Add taskmanager.memory.flink.size to common options

[dawidwys]

What is the purpose of the change

Add taskmanager.memory.flink.size to common options.

Verifying this change

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

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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Documentation

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

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[dawidwys]

cc @azagrebin @tillrohrmann

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[tillrohrmann]

Thanks for creating this PR @dawidwys. The changes look good to me. I had one minor comment which we should address before merging.

[tillrohrmann]

Adding the parenthesis with the reference to flink.size somehow reads as if one needs to configure this in order to make process.size work. I would rather suggest to add a sentence See also taskmanager.memory.flink.size for flink process mmeory size configuration.