math.c: improve docs for Math.sqrt

* math.c: [DOC] mention possibly surprising behavior of Math.sqrt
  due to floating point arithmetic; also refer to BigDecimal#sqrt.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58291 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

math.c

@@ -590,6 +590,13 @@ math_log10(VALUE unused_obj, VALUE x)
  *    #   [8, 2.82842712474619, 8.0]
  *    #   [9, 3.0, 9.0]
  *    #   [10, 3.16227766016838, 10.0]
+ *
+ *  Note that the limited precision of floating point arithmetic
+ *  might lead to surprising results:
+ *
+ *    Math.sqrt(10**46).to_i  #=> 99999999999999991611392 (!)
+ *
+ *  See also BigDecimal#sqrt.
  */
 
 static VALUE