Add a comment explaining exactly what is returned by this function

because we wasted a good amount of time today assuming that it was
returning the actual netmask.  Specifically, we were confused why it
returned 0x18 instead of 0xffffff00 for a class C subnet (the
head-smacking moment wasn't until [much] later when we converted 0x18
to decimal, which is 24.  Then the Clue Light(tm) went on).

This commit was SVN r20002.

opal/util/if.h

@@ -133,7 +133,8 @@ OPAL_DECLSPEC int opal_ifindextoaddr(int if_index, struct sockaddr*,
                                      unsigned int);
 
 /**
- *  Lookup an interface by index and return its network mask.
+ *  Lookup an interface by index and return its network mask (in CIDR
+ *  notation -- NOT the actual netmask itself!).
  *
  *  @param if_index (IN)  Interface index
  *  @param if_name (OUT)  Interface address buffer