correct comment

From-SVN: r72042

gcc/alias.c

@@ -48,7 +48,7 @@ Software Foundation, 59 Temple Place - Suite 330, Boston, MA
    different alias sets cannot alias each other, with one important
    exception.  Consider something like:
 
-     struct S {int i; double d; };
+     struct S { int i; double d; };
 
    a store to an `S' can alias something of either type `int' or type
    `double'.  (However, a store to an `int' cannot alias a `double'
@@ -604,11 +604,15 @@ new_alias_set (void)
     return 0;
 }
 
-/* Indicate that things in SUBSET can alias things in SUPERSET, but
-   not vice versa.  For example, in C, a store to an `int' can alias a
-   structure containing an `int', but not vice versa.  Here, the
-   structure would be the SUPERSET and `int' the SUBSET.  This
-   function should be called only once per SUPERSET/SUBSET pair.
+/* Indicate that things in SUBSET can alias things in SUPERSET, but that
+   not everything that aliases SUPERSET also aliases SUBSET.  For example,
+   in C, a store to an `int' can alias a load of a structure containing an
+   `int', and vice versa.  But it can't alias a load of a 'double' member
+   of the same structure.  Here, the structure would be the SUPERSET and
+   `int' the SUBSET.  This relationship is also described in the comment at
+   the beginning of this file.
+
+   This function should be called only once per SUPERSET/SUBSET pair.
 
    It is illegal for SUPERSET to be zero; everything is implicitly a
    subset of alias set zero.  */