doc: Add udp broadcast option description

Describe the option added in commit f1c167496e

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/protocols.texi

@@ -1153,6 +1153,12 @@ Set raise error timeout, expressed in microseconds.
 
 This option is only relevant in read mode: if no data arrived in more
 than this time interval, raise error.
+
+@item broadcast=@var{1|0}
+Explicitly allow or disallow UDP broadcasting.
+
+Note that broadcasting may not work properly on networks having
+a broadcast storm protection.
 @end table
 
 @subsection Examples