doc: Specifies that upgrades From Replica Sets to Multi-Sharded Insta…

…nces of cluster and advanced cluster might lead to error (#2080)

* add upgrade from replica set to sharded info

* update cluster documentation for replication spec

* align num_shard description with API

website/docs/r/advanced_cluster.html.markdown

@@ -539,6 +539,7 @@ replication_specs {
 ```
 
 * `num_shards` - (Required) Provide this value if you set a `cluster_type` of SHARDED or GEOSHARDED. Omit this value if you selected a `cluster_type` of REPLICASET. This API resource accepts 1 through 50, inclusive. This parameter defaults to 1. If you specify a `num_shards` value of 1 and a `cluster_type` of SHARDED, Atlas deploys a single-shard [sharded cluster](https://docs.atlas.mongodb.com/reference/glossary/#std-term-sharded-cluster). Don't create a sharded cluster with a single shard for production environments. Single-shard sharded clusters don't provide the same benefits as multi-shard configurations.
+If you are upgrading a replica set to a sharded cluster, you cannot increase the number of shards in the same update request. You should wait until after the cluster has completed upgrading to sharded and you have reconnected all application clients to the MongoDB router before adding additional shards. Otherwise, your data might become inconsistent once MongoDB Cloud begins distributing data across shards. To learn more, see [Convert a replica set to a sharded cluster documentation](https://www.mongodb.com/docs/atlas/scale-cluster/#convert-a-replica-set-to-a-sharded-cluster) and [Convert a replica set to a sharded cluster tutorial](https://www.mongodb.com/docs/upcoming/tutorial/convert-replica-set-to-replicated-shard-cluster).
 * `region_configs` - (Optional) Configuration for the hardware specifications for nodes set for a given regionEach `region_configs` object describes the region's priority in elections and the number and type of MongoDB nodes that Atlas deploys to the region. Each `region_configs` object must have either an `analytics_specs` object, `electable_specs` object, or `read_only_specs` object. See [below](#region_configs)
 * `zone_name` - (Optional) Name for the zone in a Global Cluster.
 

website/docs/r/cluster.html.markdown

@@ -424,7 +424,8 @@ replication_specs {
 
 **Replication Spec**  - Configuration block for multi-region cluster.
 
-* `num_shards` - (Required) Number of shards to deploy in the specified zone, minimum 1.
+* `num_shards` - (Required) Number of shards up to 50 to deploy for a sharded cluster. The resource returns 1 to indicate a replica set and values of 2 and higher to indicate a sharded cluster. The returned value equals the number of shards in the cluster.
+If you are upgrading a replica set to a sharded cluster, you cannot increase the number of shards in the same update request. You should wait until after the cluster has completed upgrading to sharded and you have reconnected all application clients to the MongoDB router before adding additional shards. Otherwise, your data might become inconsistent once MongoDB Cloud begins distributing data across shards. To learn more, see [Convert a replica set to a sharded cluster documentation](https://www.mongodb.com/docs/atlas/scale-cluster/#convert-a-replica-set-to-a-sharded-cluster) and [Convert a replica set to a sharded cluster tutorial](https://www.mongodb.com/docs/upcoming/tutorial/convert-replica-set-to-replicated-shard-cluster).
 * `id` - (Optional) Unique identifer of the replication document for a zone in a Global Cluster.
 * `regions_config` - (Optional) Physical location of the region. Each regionsConfig document describes the region’s priority in elections and the number and type of MongoDB nodes Atlas deploys to the region. You must order each regionsConfigs document by regionsConfig.priority, descending. See [Region Config](#region-config) below for more details.
 * `zone_name` - (Optional) Name for the zone in a Global Cluster.