Correct registerMarker docs

Fixes #4409

Author: Tyriar

typings/xterm.d.ts

@@ -954,8 +954,7 @@ declare module 'xterm' {
     deregisterCharacterJoiner(joinerId: number): void;
 
     /**
-     * Adds a marker to the normal buffer and returns it. If the alt buffer is
-     * active, undefined is returned.
+     * Adds a marker to the normal buffer and returns it.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */