Add links about floating point. [ruby-core:31849]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@29131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

numeric.c

@@ -533,6 +533,13 @@ num_to_int(VALUE num)
  *  <code>Float</code> objects represent inexact real numbers using
  *  the native architecture's double-precision floating point
  *  representation.
+ *
+ *  Floating point has a different arithmetic and is a inexact number.
+ *  So you should know its esoteric system. see following:
+ *  
+ *  - http://docs.sun.com/source/806-3568/ncg_goldberg.html
+ *  - http://wiki.github.com/rdp/ruby_tutorials_core/ruby-talk-faq#floats_imprecise
+ *  - http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems
  */
 
 VALUE