Call "CST" a time zone abbreviation, not a name

In documentation, call strings like "CST" time zone abbreviations, not
time zone names.  This terminology is more precise, and is what tzdb uses.
A string like "CST" is ambiguous and does not fully name a time zone.

timezone/tst-bz28707.c

@@ -37,7 +37,7 @@ do_test (void)
   tzset ();
 
   return
-    /* Sanity-check that we got the right timezone-name for DST.  For
+    /* Sanity-check that we got the right abbreviation for DST.  For
        normal time, we're likely to get "-00" (the "unspecified" marker),
        even though the POSIX timezone string says "-04".  Let's not test
        that.  */