Clarify Cue.DIMEN_UNSET is also used for size

PiperOrigin-RevId: 283559073

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

@@ -32,7 +32,7 @@ public class Cue {
   /** The empty cue. */
   public static final Cue EMPTY = new Cue("");
 
-  /** An unset position or width. */
+  /** An unset position, width or size. */
   // Note: We deliberately don't use Float.MIN_VALUE because it's positive & very close to zero.
   public static final float DIMEN_UNSET = -Float.MAX_VALUE;