Document connection management settings

See MB-43001:
Searching for max_connections in the site returns nothing, so basically is a hidden feature.

We should document:
max_connections and system_connections - see MB-32704 and MB-32791.
For example, to change the maximum number of connections to 12345:
curl -u Administrator:password -X POST localhost:8091/pools/default/settings/memcached/global --data max_connections=12345

Recommend adding to https://docs.couchbase.com/server/current/rest-api/rest-cluster-intro.html
The max connections setting should also have a link from this page, as it is a usually hit in searching about this:
https://docs.couchbase.com/server/current/learn/clusters-and-availability/size-limitations.html