Docs: Informations about shard-aware LBPs

Author: Lorak-mmk

docs/source/load-balancing/load-balancing.md

@@ -2,7 +2,7 @@
 
 ## Introduction
 
-The driver uses a load balancing policy to determine which node(s) to contact
+The driver uses a load balancing policy to determine which node(s) and shard(s) to contact
 when executing a query. Load balancing policies implement the
 `LoadBalancingPolicy` trait, which contains methods to generate a load
 balancing plan based on the query information and the state of the cluster.
@@ -15,11 +15,13 @@ using `SessionBuilder::host_filter` method.
 ## Plan
 
 When a query is prepared to be sent to the database, the load balancing policy
-constructs a load balancing plan. This plan is essentially a list of nodes to
+constructs a load balancing plan. This plan is essentially a list of pairs of node and optional shard to
 which the driver will try to send the query. The first elements of the plan are
-the nodes which are the best to contact (e.g. they might be replicas for the
+the nodes+shards which are the best to contact (e.g. they might be replicas for the
 requested data or have the best latency).
 
+In this chapter, "element" will refer to a pair of node and optional shard.
+
 ## Policy
 
 The Scylla/Cassandra driver provides a default load balancing policy (see
@@ -84,15 +86,15 @@ first element of the load balancing plan is needed, so it's usually unnecessary
 to compute entire load balancing plan. To optimize this common case, the
 `LoadBalancingPolicy` trait provides two methods: `pick` and `fallback`.
 
-`pick` returns the first node to contact for a given query, which is usually
+`pick` returns the first element to contact for a given query, which is usually
 the best based on a particular load balancing policy. If `pick` returns `None`,
 then `fallback` will not be called.
 
-`fallback`, returns an iterator that provides the rest of the nodes in the load
-balancing plan. `fallback` is called only when using the initial picked node
+`fallback`, returns an iterator that provides the rest of the elements in the load
+balancing plan. `fallback` is called only when using the initial picked element
 fails (or when executing speculatively).
 
-It's possible for the `fallback` method to include the same node that was
+It's possible for the `fallback` method to include the same element that was
 returned by the `pick` method. In such cases, the query execution layer filters
 out the picked node from the iterator returned by `fallback`.