Fix nitpicks

Fix some more nitpicks I encountered.

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/FlacStreamMetadata.java

@@ -249,7 +249,9 @@ public final class FlacStreamMetadata {
     return metadata == null ? other : metadata.copyWithAppendedEntriesFrom(other);
   }
 
-  /** Returns */
+  /**
+   * Returns a new Metadata instance created from {@code vorbisComments} and {@code pictureFrames}
+   */
   @Nullable
   private static Metadata concatenateVorbisMetadata(
       List<String> vorbisComments,

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/VorbisUtil.java

@@ -265,7 +265,7 @@ public final class VorbisUtil {
   }
 
   /**
-   * Builds a metadata instance from Vorbis comments.
+   * Builds a metadata instance from a list of Vorbis Comments.
    *
    * METADATA_BLOCK_PICTURE comments will be transformed into picture frames.
    * All others will be transformed into vorbis comments.