kcmkwin/xwayland: add an explanation on top

These settings are highly technical, and users are likely to feel
completely lost or intimidated when they see them if they're just
browsing around. Let's add some text to explain what the heck this is
for.

src/kcmkwin/kwinxwayland/package/contents/ui/main.qml

@@ -43,6 +43,19 @@ KCM.SimpleKCM {
         id: column
         spacing: Kirigami.Units.smallSpacing
 
+        QQC2.Label {
+            Layout.fillWidth: true
+            Layout.margins: Kirigami.Units.gridUnit
+            text: i18n("Legacy X11 apps require the ability to read keystrokes typed in other apps for features that are activated using global keyboard shortcuts. This is disabled by default for security reasons. If you need to use such apps, you can chose your preferred balance of security and functionality here.")
+            wrapMode: Text.Wrap
+        }
+
+        Kirigami.Separator {
+            Layout.fillWidth: true
+            Layout.leftMargin: Kirigami.Units.gridUnit
+            Layout.rightMargin: Kirigami.Units.gridUnit
+        }
+
         Kirigami.FormLayout {
             id: form