Improve InitialOffsetProvider API documentation. (Azure#20473)

* Improve InitialOffsetProvider API documentation.

* Update EventProcessorOptions.cs

Author: serkantkaraca

sdk/eventhub/Microsoft.Azure.EventHubs.Processor/src/EventProcessorOptions.cs

@@ -82,7 +82,7 @@ public bool EnableReceiverRuntimeMetric
         /// <summary>
         /// Gets or sets a delegate which is used to get the initial position for a given partition to create <see cref="PartitionReceiver"/>.
         /// Delegate is invoked by passing in PartitionId and then user can return <see cref="PartitionReceiver"/> for receiving messages.
-        /// This is only used when <see cref="Lease.Offset"/> is not provided and receiver is being created for the very first time.
+        /// This is only used when a checkpoint cannot be found for the associated partition.
         /// </summary>
         public Func<string, EventPosition> InitialOffsetProvider { get; set; }
 
@@ -114,4 +114,4 @@ internal void NotifyOfException(string hostname, string partitionId, Exception e
             }
         }
     }
-}
+}