doc/filters: some more details and modified example to zmq/azmq

Info about default value of bind_address option and its abbreviated
version (b). Example modified to have named instanced filter and to show
its use.
Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -221,6 +221,7 @@ Here is a BNF description of the filtergraph syntax:
 @var{FILTERGRAPH}      ::= [sws_flags=@var{flags};] @var{FILTERCHAIN} [;@var{FILTERGRAPH}]
 @end example
 
+@anchor{filtergraph escaping}
 @section Notes on filtergraph escaping
 
 Filtergraph description composition entails several levels of
@@ -20270,7 +20271,7 @@ filters in the filtergraph.
 
 @code{zmq} and @code{azmq} work as a pass-through filters. @code{zmq}
 must be inserted between two video filters, @code{azmq} between two
-audio filters.
+audio filters. Both are capable to send messages to any filter type.
 
 To enable these filters you need to install the libzmq library and
 headers and configure FFmpeg with @code{--enable-libzmq}.
@@ -20280,7 +20281,10 @@ For more information about libzmq see:
 
 The @code{zmq} and @code{azmq} filters work as a libzmq server, which
 receives messages sent through a network interface defined by the
-@option{bind_address} option.
+@option{bind_address} (or the abbreviation "@option{b}") option.
+Default value of this option is @file{tcp://localhost:5555}. You may
+want to alter this value to your needs, but do not forget to escape any
+':' signs (see @ref{filtergraph escaping}).
 
 The received message must be in the form:
 @example
@@ -20288,7 +20292,10 @@ The received message must be in the form:
 @end example
 
 @var{TARGET} specifies the target of the command, usually the name of
-the filter class or a specific filter instance name.
+the filter class or a specific filter instance name. The default
+filter instance name uses the pattern @samp{Parsed_<filter_name>_<index>},
+but you can override this by using the @samp{filter_name@@id} syntax
+(see @ref{Filtergraph syntax}).
 
 @var{COMMAND} specifies the name of the command for the target filter.
 
@@ -20310,14 +20317,17 @@ will send a reply to the client, adopting the format:
 Look at @file{tools/zmqsend} for an example of a zmq client which can
 be used to send commands processed by these filters.
 
-Consider the following filtergraph generated by @command{ffplay}
+Consider the following filtergraph generated by @command{ffplay}.
+In this example the last overlay filter has an instance name. All other
+filters will have default instance names.
+
 @example
 ffplay -dumpgraph 1 -f lavfi "
 color=s=100x100:c=red  [l];
 color=s=100x100:c=blue [r];
 nullsrc=s=200x100, zmq [bg];
-[bg][l]   overlay      [bg+l];
-[bg+l][r] overlay=x=100 "
+[bg][l]   overlay     [bg+l];
+[bg+l][r] overlay@@my=x=100 "
 @end example
 
 To change the color of the left side of the video, the following
@@ -20331,6 +20341,12 @@ To change the right side:
 echo Parsed_color_1 c pink | tools/zmqsend
 @end example
 
+To change the position of the right side:
+@example
+echo overlay@@my x 150 | tools/zmqsend
+@end example
+
+
 @c man end MULTIMEDIA FILTERS
 
 @chapter Multimedia Sources