stream_lavf: expose concat://

Apparently there's an use for this; see #1178.

I won't redocument obscure FFmpeg features, so add a hint to the
manpage that some protocols are documented in FFmpeg instead.

DOCS/man/mpv.rst

@@ -467,6 +467,10 @@ PROTOCOLS
     be specified. mpv will never attempt to guess whether a filename is
     actually a network address. A protocol prefix is always required.
 
+    Note that not all prefixes are documented here. Undocumented prefixes are
+    either aliases to documented protocols, or are just reidrections to
+    protocols implemented and documented in FFmpeg.
+
 ``-``
     Play data from stdin.
 

stream/stream_lavf.c

@@ -339,6 +339,7 @@ const stream_info_t stream_info_ffmpeg_unsafe = {
   .open = open_f,
   .protocols = (const char *const[]){
      "lavf", "ffmpeg", "udp", "ftp", "tcp", "tls", "unix", "sftp", "md5",
+     "concat",
      NULL },
   .can_write = true,
 };