Add a note explaining 1GB requirement for minimum bucket memory

This relates to the following page:

https://docs.couchbase.com/server/current/learn/buckets-memory-and-storage/storage-engines.html

In the table "Couchstore and Magma at a Glance", the first item is for minimum bucket memory quota. For Magma we need a note to explain the 1 GB. The note should say the following:

"Magma's minimum memory requirement is higher at 1GB per node due to the more involved data structures it has to maintain. We are working on reducing this to a lower number in a future release."

Also "Minimum bucket memory quota" should be clarified to "Minimum bucket memory quota per node".

Sarath Lakshman Perry Krug Roi Katz  fyi in case you want to add/modify.