Add information to Float

The current reference manual does not clearly show that Float() can take hexadecimalfloating point format as an argument.
According to https://github.com/ruby/ruby/tree/v1_9_2_381, Float() has supported hexadecimal floating point format since Ruby 1.9.2.
The reference manual of 1.9.2 and later versions will show the exmaple with an argument of hexadecimal floating format.