[DOCS] Added documentation for SameShardAllocationDecider

Closes elastic#4615

Author: spinscale

docs/reference/modules/cluster.asciidoc

@@ -56,6 +56,10 @@ The following settings may be used:
       setting, mainly make sense when using it dynamically using the 
       cluster update settings API.
 
+`cluster.routing.allocation.same_shard.host`::
+      Prevents that multiple instances of the same shard are allocated
+      on a single host. Defaults to `false`. This setting only applies 
+      if multiple nodes are started on the same machine.
 
 `indices.recovery.concurrent_streams`::
        The number of streams to open (on a *node* level) to recover a 

src/main/java/org/elasticsearch/cluster/routing/allocation/decider/SameShardAllocationDecider.java

@@ -27,10 +27,10 @@
 import org.elasticsearch.common.settings.Settings;
 
 /**
- * An allocation decider that prevents the more than prevents multiple instances
- * of the same shard to be allocated on a single <tt>host</tt>. The cluster setting can
- * be modified in real-time by updating the {@value #SAME_HOST_SETTING} value of
- * cluster setting API. The default is <code>false</code>.
+ * An allocation decider that prevents multiple instances of the same shard to be
+ * allocated on a single <tt>host</tt>. The cluster setting can be modified in
+ * real-time by updating the {@value #SAME_HOST_SETTING} value of cluster setting
+ * API. The default is <code>false</code>.
  * <p>
  * Note: this setting only applies if multiple nodes are started on the same
  * <tt>host</tt>. Multiple allocations of the same shard on the same <tt>node</tt> are