[trac1155] a comment about Solaris and DS added

src/lib/dns/rrtype-placeholder.h

@@ -22,6 +22,7 @@
 
 #include <exceptions/exceptions.h>
 
+// Solaris x86 defines DS in <sys/regset.h>, which gets pulled in by Boost
 #if defined(__sun) && defined(DS)
 # undef DS
 #endif