Add missing javadoc to new ExoPlayer.Builder constructors

Should have been part of
https://github.com/google/ExoPlayer/commit/98200c2692ba007ba0b177d7b285b957dc08ff93

#minor-release

PiperOrigin-RevId: 405880982

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java

@@ -457,10 +457,30 @@ public interface ExoPlayer extends Player {
           new DefaultMediaSourceFactory(context, new DefaultExtractorsFactory()));
     }
 
+    /**
+     * Creates a builder with a custom {@link MediaSourceFactory}.
+     *
+     * <p>See {@link #Builder(Context)} for a list of default values.
+     *
+     * @param context A {@link Context}.
+     * @param mediaSourceFactory A factory for creating a {@link MediaSource} from a {@link
+     *     MediaItem}.
+     */
     public Builder(Context context, MediaSourceFactory mediaSourceFactory) {
       this(context, new DefaultRenderersFactory(context), mediaSourceFactory);
     }
 
+    /**
+     * Creates a builder with a custom {@link RenderersFactory} and {@link MediaSourceFactory}.
+     *
+     * <p>See {@link #Builder(Context)} for a list of default values.
+     *
+     * @param context A {@link Context}.
+     * @param renderersFactory A factory for creating {@link Renderer Renderers} to be used by the
+     *     player.
+     * @param mediaSourceFactory A factory for creating a {@link MediaSource} from a {@link
+     *     MediaItem}.
+     */
     public Builder(
         Context context, RenderersFactory renderersFactory, MediaSourceFactory mediaSourceFactory) {
       this(