Provide better explanation of "nodes" clause in create index

"nodes" clause allows a superset of node list to be provided during Create Index. Index planner choses from this superset of nodes to place the index and its replica.

This is particularly important to explain for partitioned index e.g. if an index has 8 partitions and 8 nodes are specified in the Create Index(cluster may have 10 index nodes), it doesn't necessarily mean that 1 partition would be placed on each node. Rather, index planner would consider the resource usage on each of these 8 nodes and may chose to place the the partitions on 4 of these nodes.