add documentation of the relationship between cache-max-memory-size and cache-snapshot-memory-size

This issue asks about the connection between cache-max-memory-size and cache-snapshot-memory-size.

https://community.influxdata.com/t/is-there-a-any-recommendation-for-tuning-cache-memory-size/16102

We should add this to the page about optimizing writes (or maybe somewhere else).

```
Perhaps your data rate is faster than you can snapshot, given the difference between snapshot memory size and cache maximum size? As I understand the doc, the cache is written out out when it hits storage-cache-snapshot-memory-size . So, you need a sufficient size difference between the max and the snapshot size, so that you don't overflow the cache while snapshotting.
```

##### Relevant URLs
https://docs.influxdata.com/influxdb/v2.0/write-data/best-practices/optimize-writes/