Improve documentation of tidb-server oom behavior

[kolbe]

Change Request

This repository is ONLY used to solve issues related to DOCS.
For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

The current explanations of TiDB memory management and behavior when a query uses a large amount of memory are incomplete and confusing.

2. Describe your suggestion or addition.

https://docs.pingcap.com/tidb/dev/configure-memory-usage discusses configuration and behavior of some options, but it does not discuss the circumstances or operation of oom-use-tmp-storage, which controls whether large operations can be "spilled" to disk.

https://docs.pingcap.com/tidb/dev/tidb-configuration-file#oom-use-tmp-storage should be updated to clarify what "some operators" means. Which operators can use temporary storage and which can't? Can the location of the temporary storage be configured? How can usage of this temporary storage be monitored?

3. Provide some reference materials (documents, websites, etc) if you could.

@tiancaiamao reports:

• sort and merge join and hash join may use conf.OOMUseTmpStorage
• aggregates cannot spill to disk

[ran-huang]

@SunRunAway PTAL. Could you update https://docs.pingcap.com/tidb/dev/tidb-configuration-file#oom-use-tmp-storage?

[TomShawn]

@XuHuaiyu Would you help take a look?