[FLINK-14123]Update help docs for OutOfMemoryError

[liupc]

This PR add docs about handling OutOfMemoryError, it's related to https://issues.apache.org/jira/browse/FLINK-14123

[infoverload]

Suggested change

	In Fink 1.9.0 and prior version, the managed memory fraction of taskmanager is controlled by `taskmanager.memory.fraction`,
	and with 0.7 as the default value. This sometimes will cause OOMs due to the fact that the default value of
	JVM parameter `NewRatio` is 2, which means the old generation occupied only 2/3 (0.66) of the heap memory.
	In Flink 1.9.0 and prior versions, the managed memory fraction of the taskmanager is controlled by `taskmanager.memory.fraction`,
	with 0.7 as the default value. This will sometimes cause OOMs because the default value of the
	JVM parameter `NewRatio` is 2, which means that the old generation occupied only 2/3 (0.66) of the heap memory.