doc/faq: improve concatenation entry.

doc/faq.texi

@@ -265,12 +265,19 @@ the other; it can be done using the
 
 There are several solutions, depending on the exact circumstances.
 
-@subsection Concatenating using filters
+@subsection Concatenating using the concat @emph{filter}
 
-FFmpeg has a @url{http://ffmpeg.org/ffmpeg.html#concat-1, @code{concat}}
-filter designed specifically for that, with examples in the documentation.
+FFmpeg has a @url{http://ffmpeg.org/ffmpeg-filters.html#concat,
+@code{concat}} filter designed specifically for that, with examples in the
+documentation. This operation is recommended if you need to re-encode.
 
-@subsection Concatenating at the file level
+@subsection Concatenating using the concat @emph{demuxer}
+
+FFmpeg has a @url{http://www.ffmpeg.org/ffmpeg-formats.html#concat,
+@code{concat}} demuxer which you can use when you want to avoid a re-encode and
+your format doesn't support file level concatenation.
+
+@subsection Concatenating using the concat @emph{protocol} (file level)
 
 A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to concatenate
 video by merely concatenating the files them.