xtrlock: add page (#2501)

pages/linux/xtrlock.md

@@ -0,0 +1,15 @@
+# xtrlock
+
+> Lock the X display until the user supplies their password.
+
+- Lock the display and show a padlock instead of the cursor:
+
+`xtrlock`
+
+- Display a blank screen as well as the padlock cursor:
+
+`xtrlock -b`
+
+- Fork the xtrlock process and return immediately:
+
+`xtrlock -f`