more detailed note for vacpp (iostreams)

[SVN r42689]

status/explicit-failures-markup.xml

@@ -2025,7 +2025,9 @@
             <test name="restrict_test"/>
             <toolset name="vacpp"/>
             <note author="Jonathan Turkanis" date="06 Jan 2008">
-                "restrict" is treated as a keyword (as in C99)
+                "restrict" is treated as a keyword on this platform (as in C99);
+                use the alias "slice" instead, defined in 
+                "boost/iostreams/slice.hpp."
             </note>
         </mark-expected-failures>
         <mark-expected-failures>