Should suggest using Disk I/O Optimized mode for Magma

This affects two pages:

• https://docs-staging.couchbase.com/server/7.1/learn/buckets-memory-and-storage/storage-settings.html#threading
• https://docs-staging.couchbase.com/server/7.1/manage/manage-settings/general-settings.html#data-settings

For the first page (Threading discussion in Storage Settings):

Current para is:

A high thread-allocation may improve performance on systems whose hardware-resources are commensurately supportive (for example, where the number of CPU cores is high). In particular, a high number of writer threads on such systems may significantly optimize the performance of durable writes: see Durability, for information.

Lets add the following sentences at end of above para:

A high number of reader and writer threads will benefit disk based workloads that require high throughput especially when using high end disk drives such as NVMe SSDs. This is likely to be the case when using Magma as the storage engine. In this case it is best to choose 'Disk i/o optimized' mode for Reader and Writer thread settings.

It will be good to link to the Data Settings from the mention of 'Disk i/o optimized'

For the second page (Data settings section in  General Settings page):

Add the following sentence here as well:

A high number of reader and writer threads will benefit disk based workloads that require high throughput especially when using high end disk drives such as NVMe SSDs. This is likely to be the case when using Magma as the storage engine. In this case it is best to choose 'Disk i/o optimized' mode for Reader and Writer thread settings.