doc: 增加 ShowSelectedValue 说明

Author: ArgoZhang

src/BootstrapBlazor.Server/Components/Samples/DateTimeRanges.razor.cs

@@ -182,6 +182,14 @@ private static List<AttributeItem> GetAttributes() =>
             DefaultValue = "false"
         },
         new()
+        {
+            Name = nameof(DateTimeRange.ShowSelectedValue),
+            Description = "Whether to show the selected value",
+            Type = "bool",
+            ValueList = "true|false",
+            DefaultValue = "false"
+        },
+        new()
         {
             Name = "ShowToday",
             Description = "Whether to show today shortcut button",