zebra: Add some documentation on when zserv_open should be used

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

zebra/zserv.c

@@ -937,6 +937,10 @@ void zserv_close(void)
 /*
  * Open zebra's ZAPI listener socket. This is done early during startup,
  * before zebra is ready to listen and accept client connections.
+ *
+ * This function should only ever be called from the startup pthread
+ * from main.c.  If it is called multiple times it will cause problems
+ * because it causes the zsock global variable to be setup.
  */
 void zserv_open(const char *path)
 {

zebra/zserv.h

@@ -262,6 +262,9 @@ extern void zserv_close(void);
  *
  * path
  *    where to place the Unix domain socket
+ *
+ * This function *should* only ever be called from
+ * main() and only every from 1 pthread.
  */
 extern void zserv_open(const char *path);