Minor ObjC header updates.

BUG=

Review URL: https://codereview.webrtc.org/1845133002

Cr-Commit-Position: refs/heads/master@{#12183}
diff --git a/webrtc/api/objc/RTCVideoFrame.h b/webrtc/api/objc/RTCVideoFrame.h
index 8ed23ba..8061047 100644
--- a/webrtc/api/objc/RTCVideoFrame.h
+++ b/webrtc/api/objc/RTCVideoFrame.h
@@ -12,6 +12,7 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+// RTCVideoFrame is an ObjectiveC version of cricket::VideoFrame.
 @interface RTCVideoFrame : NSObject
 
 /** Width without rotation applied. */