HBASE-25291 Document how to enable the meta replica load balance mode for the client and clean up around hbase:meta read replicas

hbase-client/src/main/java/org/apache/hadoop/hbase/client/CatalogReplicaLoadBalanceSelector.java
hbase-client/src/main/java/org/apache/hadoop/hbase/client/CatalogReplicaLoadBalanceSelectorFactory.java
hbase-client/src/main/java/org/apache/hadoop/hbase/client/CatalogReplicaLoadBalanceSimpleSelector.java
hbase-client/src/main/java/org/apache/hadoop/hbase/client/CatalogReplicaMode.java
 Make the leading license comment opener align on all these new
 files.... one star rather than two.

hbase-server/src/main/java/org/apache/hadoop/hbase/replication/regionserver/CatalogReplicationSource.java
 Add some comment better situating the catalogreplicationsource
 specialization.

hbase-server/src/main/java/org/apache/hadoop/hbase/replication/regionserver/ReplicationSourceManager.java
 Add comment how this differs from replicationsource factory and why no
 need to keep offset up in zk.

src/main/asciidoc/_chapters/architecture.adoc
 Clean up the read replica of META doc. Introduce the new read replica
 liveness on META and the new 'LoadBalance' feature (with warnings).
 General edit.
7 files changed