Update Complex#infinite? documenation

to state what it really does.

[Fix GH-1848]

From: Christian Bruckmayer <cbruckmayer@suse.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62921 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

complex.c

@@ -1273,10 +1273,8 @@ rb_complex_finite_p(VALUE self)
  * call-seq:
  *    cmp.infinite?  ->  nil or 1
  *
- * Returns values corresponding to the value of +cmp+'s magnitude:
- *
- * +finite+::    +nil+
- * ++Infinity+:: ++1+
+ * Returns +1+ if +cmp+'s real or imaginary value is an infinite number,
+ * otherwise returns +nil+.
  *
  *  For example:
  *