Add description for join enumeration for CBO

In Neo we added join enumeration for CBO, this is the ability for the cost-based optimizer to consider different join ordering to come up with the query plan. In RBO, as well as CBO in 7.0, the order of the joins is the order that is specified in the query itself, no join reordering is considered. In Neo, CBO enumerates different join ordering to come up with a query plan.

We can potentially add a new section under:

https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/cost-based-optimizer.html#advantages

the new section just need to describe the fact that different join orders are considered by CBO, something like:

Join Order

With the legacy rule-based optimizer, the order of the joins is the order they are specified in the query, no reordering of joins is considered. The cost-based optimizer is able to consider different join orders and choose a query plan based on cost.