[ENB] enb.conf.example: Mention WRITE_SIB_PCAP

The documentation talks about a MAC protocol trace, but in fact
it is only a protocol trace of dedicated channels.  There's a related
define in the source code, but that's not documented.

Let's at least make the user aware that the MAC pcap trace is
for dedicated channels only, unless he uses that #define.

srsenb/enb.conf.example

@@ -89,6 +89,11 @@ rx_gain = 40
 # add an entry with DLT=147, Payload Protocol=mac-lte-framed.
 # For more information see: https://wiki.wireshark.org/MAC-LTE
 #
+# Please note that this setting will by default only capture MAC
+# frames on dedicated channels, and not SIB.  You have to build with
+# WRITE_SIB_PCAP enabled in srsenb/src/stack/mac/mac.cc if you want
+# SIB to be part of the MAC pcap file.
+#
 # enable:   Enable MAC layer packet captures (true/false)
 # filename: File path to use for packet captures
 #####################################################################