Add parse-metadata and video+audio extraction recipes

YTDL_OPTIONS-Cookbook.md

@@ -62,3 +62,30 @@ global, define it as a named preset via `YTDL_OPTIONS_PRESETS`.
 
 Note: postprocessors only run on **success** — there is currently no hook for
 failed downloads.
+
+## Using --parse-metadata (MetadataParser) from JSON
+
+yt-dlp's `--parse-metadata` normally builds Python tuples that JSON can't
+express. Use the `MetadataFromField` postprocessor instead — it takes plain
+`"FROM:TO"` strings and behaves identically. For example, to clear the
+description (the equivalent of `--parse-metadata ":(?P<meta_description>)"`):
+
+    YTDL_OPTIONS={"postprocessors":[{"key":"MetadataFromField","formats":[":(?P<meta_description>)"],"when":"pre_process"}]}
+
+Each entry in `formats` is one `FROM:TO` expression, exactly as you would
+write it on the yt-dlp command line.
+
+## Download video and extract audio in one go
+
+`keepvideo` plus `FFmpegExtractAudio` gives you both the video file and an
+audio file from a single download:
+
+    YTDL_OPTIONS={"keepvideo":true,"postprocessors":[{"key":"FFmpegExtractAudio","preferredcodec":"mp3"}]}
+
+To convert the video container instead (e.g. everything as MKV):
+
+    YTDL_OPTIONS={"postprocessors":[{"key":"FFmpegVideoConvertor","preferedformat":"mkv"}]}
+
+(Note: yt-dlp's option is really spelled `preferedformat`, with one `r`.)
+Define these as named presets via `YTDL_OPTIONS_PRESETS` to pick them
+per-download in the UI instead of applying them globally.