docs: update and correct configuration documentation

docs/DOWNLOAD_CONFIG.md

@@ -54,7 +54,9 @@ Options:
 - `curl_impersonate` - <https://github.com/yifeikong/curl-impersonate> (via <https://github.com/yifeikong/curl_cffi>)
 - `n_m3u8dl_re` - <https://github.com/nilaoda/N_m3u8DL-RE>
 
-Note that aria2c can reach the highest speeds as it utilizes threading and more connections than the other downloaders. However, aria2c can also be one of the more unstable downloaders. It will work one day, then not another day. It also does not support HTTP(S) proxies while the other downloaders do.
+Note that aria2c can reach the highest speeds as it utilizes threading and more connections than the other downloaders. However, aria2c can also be one of the more unstable downloaders. It will work one day, then not another day. It also does not support HTTP(S) proxies natively (non-HTTP proxies are bridged via pproxy).
+
+Note that `n_m3u8dl_re` will automatically fall back to `requests` for track types it does not support, specifically: direct URL downloads, Subtitle tracks, and Attachment tracks.
 
 Example mapping:
 
@@ -72,10 +74,12 @@ The `default` entry is optional. If omitted, `requests` will be used for service
 
 ## n_m3u8dl_re (dict)
 
-Configuration for N_m3u8DL-RE downloader. This downloader is particularly useful for HLS streams.
+Configuration for N_m3u8DL-RE downloader. This downloader supports HLS, DASH, and ISM (Smooth Streaming) manifests.
+It will automatically fall back to the `requests` downloader for unsupported track types (direct URLs, subtitles, attachments).
 
 - `thread_count`
-  Number of threads to use for downloading. Default: Uses the same value as max_workers from the command.
+  Number of threads to use for downloading. Default: Uses the same value as max_workers from the command
+  (which defaults to `min(32,(cpu_count+4))`).
 - `ad_keyword`
   Keyword to identify and potentially skip advertisement segments. Default: `None`
 - `use_proxy`
@@ -83,6 +87,9 @@ Configuration for N_m3u8DL-RE downloader. This downloader is particularly useful
 - `retry_count`
   Number of times to retry failed downloads. Default: `10`
 
+N_m3u8DL-RE also respects the `decryption` config setting. When content keys are provided, it will use
+the configured decryption engine (`shaka` or `mp4decrypt`) and automatically locate the corresponding binary.
+
 For example,
 
 ```yaml
@@ -140,6 +147,57 @@ sub_format: vtt
 
 ---
 
+## subtitle (dict)
+
+Configuration for subtitle processing and conversion.
+
+- `conversion_method`
+  Method to use for converting subtitles between formats. Default: `"auto"`
+  - `"auto"` — Smart routing: uses subby for WebVTT/SAMI, pycaption for others.
+  - `"subby"` — Always use subby with advanced processing.
+  - `"pycaption"` — Use only pycaption library (no SubtitleEdit, no subby).
+  - `"subtitleedit"` — Prefer SubtitleEdit when available, fall back to pycaption.
+  - `"pysubs2"` — Use pysubs2 library (supports SRT/SSA/ASS/WebVTT/TTML/SAMI/MicroDVD/MPL2/TMP).
+- `sdh_method`
+  Method to use for SDH (hearing impaired) stripping. Default: `"auto"`
+  - `"auto"` — Try subby (SRT only), then SubtitleEdit (if available), then subtitle-filter.
+  - `"subby"` — Use subby library (SRT only).
+  - `"subtitleedit"` — Use SubtitleEdit tool (Windows only, falls back to subtitle-filter).
+  - `"filter-subs"` — Use subtitle-filter library directly.
+- `strip_sdh`
+  Automatically create stripped (non-SDH) versions of SDH subtitles. Default: `true`
+- `convert_before_strip`
+  Auto-convert VTT/other formats to SRT before using subtitle-filter for SDH stripping.
+  Ensures compatibility when subtitle-filter is used as fallback. Default: `true`
+- `preserve_formatting`
+  Preserve original subtitle formatting (tags, positioning, styling).
+  When `true`, skips pycaption processing for WebVTT files to keep tags like `<i>`, `<b>`,
+  positioning intact. Combined with no `sub_format` setting, ensures subtitles remain in
+  their original format. Default: `true`
+- `output_mode`
+  Output mode for subtitles. Default: `"mux"`
+  - `"mux"` — Embed subtitles in MKV container only.
+  - `"sidecar"` — Save subtitles as separate files only.
+  - `"both"` — Embed in MKV and save as sidecar files.
+- `sidecar_format`
+  Format for sidecar subtitle files when `output_mode` is `"sidecar"` or `"both"`. Default: `"srt"`
+  Options: `srt`, `vtt`, `ass`, `original` (keep current format).
+
+For example,
+
+```yaml
+subtitle:
+  conversion_method: auto
+  sdh_method: auto
+  strip_sdh: true
+  convert_before_strip: true
+  preserve_formatting: true
+  output_mode: mux
+  sidecar_format: srt
+```
+
+---
+
 ## decryption (str | dict)
 
 Choose what software to use to decrypt DRM-protected content throughout unshackle where needed.