docs: document .ts-only audio pacing limitation in videoin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/videoin/main.cpp

@@ -12,6 +12,9 @@ class VideoInNode : public dmf::NodeBase {
         if (filename.empty()) { log("config missing 'file'"); return; }
         log("file: %s", filename.c_str());
 
+        // Audio pacing assumes steady fixed-size chunks from the demuxer (e.g. 1024-sample AAC
+        // packets in MPEG-TS). Containers like MP4/MKV can deliver audio in large bursts, which
+        // would require a separate metering buffer to pace correctly. Stick to .ts for now.
         dmf::VideoReader video_reader(filename);
         if (!video_reader.has_video && !video_reader.has_audio) {
             log("no video or audio stream found"); return;