Add info on disk spilling to Production Checklist

Artem Ervits (dbist) commented:

## Production Checklist
Add information on disk spilling, currently only available in `cockroach start` based on `--temp-dir` which is very easy to ignore.

## Description
In times when workload includes complex queries and there's a lot of disk spill based on the `sql.distsql.temp_storage.workmem` metric, it is advisable to tune if not separate out the `--temp-dir` directory to a dedicated disk away from `--store` disk. More research is needed but this is an area worthy of investigation further.

See https://cockroachlabs.slack.com/archives/CJGSARZJR/p1614697210030500

## Outline

## Expected Audience

CC: @johnrk @a-entin @holtrdan

Jira Issue: DOC-1004