[Doc] Make UDP file options clearer

Issue #332.

Change-Id: I3a2b1e1e0525eaba5bb657ce0481d96ceafb4e89

docs/source/options/udp_file_options.rst

@@ -1,23 +1,30 @@
 UDP file options
 ^^^^^^^^^^^^^^^^
 
-UDP file is of the form udp://ip:port[?options]. Here is the list of supported
-options:
+UDP file is of the form::
+
+    udp://{ip}:{port}[?{option}[&{option}]...]
+
+Here is the list of supported options:
 
 :reuse=0|1:
 
     Allow or disallow reusing UDP sockets.
 
-:interface=<addr>:
+:interface={addr}:
 
     Multicast group interface address. Only the packets sent to this address are
     received. Default to "0.0.0.0" if not specified.
 
-:source=<addr>:
+:source={addr}:
 
     Multicast source ip address. Only the packets sent from this source address
-    are received. Defaults to "0.0.0.0" if not specified.
+    are received. Enables Source Specific Multicast (SSM) if set.
 
-:timeout=<microseconds>:
+:timeout={microseconds}:
 
     UDP timeout in microseconds.
+
+Example::
+
+    udp://224.1.2.30:88?interface=10.11.12.13&reuse=1