Fix about_* topic help issues and a few tab completion regressions (#4014)

test/powershell/engine/Help/HelpSystem.Tests.ps1

@@ -200,6 +200,12 @@ Describe "Validate about_help.txt under culture specific folder works" -Tags @('
         $help.count | Should Be 1
         $help | Should BeExactly "Hello"
     }
+
+    It "Get-Help for about_Variable should return only one help object" {
+
+        $help = Get-Help about_Variables
+        $help.count | Should Be 1
+    }
 }
 
 Describe "Get-Help should find help info within help files" -Tags @('CI', 'RequireAdminOnWindows') {