improve and examples for Float.parse/1 and String.to_float

fixes issue #2869
It fails the same way when I create a test for it.

    test :to_float do
        #assert_raise ArgumentError, fn -> String.to_float("3") end
        three = "3"
        number = String.to_float(three)
        assert_raise ArgumentError, fn -> number end
    end

Shell:

    lib/elixir/test/elixir/string_test.exs:498: warning: this expression will fail with ArgumentError
    ................

    1) test to_float (StringTest)
       lib/elixir/test/elixir/string_test.exs:495
       ** (ArgumentError) argument error
       stacktrace:
         lib/elixir/test/elixir/string_test.exs:498

Should I create an issue for this, maybe it can be fixed in the future.