add doc to "network" sensor explaining #interfaces hard-limit in use

doc/network.md

+This sensor can autodetect interfaces in use by giving the special
+interface name "X".  But the total number of interfaces it can autodetect
+is currently under a hard-limit.  This hard-limit can be changed by
+modifying this line in `src/network.c`:
+
+```c
+#define NB_MAX_DEV 8
+```