add comment.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39226 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/socket/extconf.rb

@@ -19,8 +19,8 @@ when /i386-os2_emx/
   have_library("socket", "socket")
 else
   test_func = "socket"
-  have_library("nsl", "t_open")
-  have_library("socket", "socket")
+  have_library("nsl", "t_open") # SunOS
+  have_library("socket", "socket") # SunOS
 end
 
 unless $mswin or $bccwin or $mingw