doc/protocols: document experimental mutli-client api

Signed-off-by: Stephan Holljes <klaxa1337@googlemail.com>

doc/protocols.texi

@@ -304,6 +304,8 @@ autodetection in the future.
 If set to 1 enables experimental HTTP server. This can be used to send data when
 used as an output option, or read data from a client with HTTP POST when used as
 an input option.
+If set to 2 enables experimental mutli-client HTTP server. This is not yet implemented
+in ffmpeg.c or ffserver.c and thus must not be used as a command line option.
 @example
 # Server side (sending):
 ffmpeg -i somefile.ogg -c copy -listen 1 -f ogg http://@var{server}:@var{port}