Tweak Cue.line javadoc again to reflect WebVTT spec

It's interesting WebVTT explicitly handles line & position differently
in horizontal-rl/lr and vertical-lr/rl contexts. position is always
measured from the left of the viewport, even for rtl text, but line in
vertical-rl is measured from the right of the viewport.

We don't have to make Cue match WebVTT (I can go change the WebVTT
decoding instead) but it seems a reasonable 'default' to follow.

PiperOrigin-RevId: 304353900

library/core/src/main/java/com/google/android/exoplayer2/text/Cue.java

@@ -154,9 +154,10 @@ public final class Cue {
    * <ul>
    *   <li>For {@link #TYPE_UNSET} (i.e. horizontal), this is the vertical position relative to the
    *       top of the viewport.
-   *   <li>For {@link #VERTICAL_TYPE_LR} and {@link #VERTICAL_TYPE_RL} (i.e. vertical), this is the
+   *   <li>For {@link #VERTICAL_TYPE_LR} this is the horizontal position relative to the left of the
-   *       horizontal position relative to the left of the viewport. Note that positioning is
+   *       viewport.
-   *       relative to the left of the viewport even in the case of vertical-growing-left text.
+   *   <li>For {@link #VERTICAL_TYPE_RL} this is the horizontal position relative to the right of
+   *       the viewport.
    * </ul>
    */
   public final float line;