docs: document base ffmpeg arguments passed before preset ffargs

Both the README and the man page now explain that the script always
passes -map 0, -map_metadata 0, -map_chapters 0, and -c copy to
ffmpeg before appending the preset's ffargs array. This context is
important for preset authors, who only need to override the streams
they intend to re-encode.

Also fix a minor blockquote rendering issue in the README (missing
blank line before the fenced code block in the PATH note).

1 files changed, 20 insertions, 0 deletions

diff --git a/transcode.sh.1 b/transcode.sh.1
index 9001e8a..fe38c21 100644
--- a/transcode.sh.1
+++ b/transcode.sh.1
@@ -158,6 +158,26 @@ The variable
 is available to presets and contains the automatically selected output
 pixel format string.
 .IP
+Before the preset's
+.B ffargs
+are appended, the script always passes the following base arguments to
+.BR ffmpeg (1):
+.RS
+.nf
+\-map 0          # include all streams from the input
+\-map_metadata 0 # copy all container\-level metadata
+\-map_chapters 0 # copy all chapter markers
+\-c copy         # default to stream copy for every stream
+.fi
+.RE
+.IP
+A preset therefore only needs to specify the streams it wishes to
+re-encode (typically
+.BR \-c:v:0 )
+and any associated encoder options. All other streams — audio,
+subtitles, attachments — are passed through unchanged unless the preset
+explicitly overrides them.
+.IP
 Example preset:
 .RS
 .nf