Reword Couchstore description on Storage Engines page

This regards the following section:

https://docs-staging.couchbase.com/server/7.1/learn/buckets-memory-and-storage/storage-engines.html#couchstore

Let's replace the para in this section with the following:

Couchstore is the default bucket storage engine that has been in use for more than ten years. It is optimized for small to medium datasets with simple data access use cases on low end hardware (the minimum bucket size for Couchstore backend is 100 MB). If you have a small data set that can fit in memory, then you should consider using Couchstore.