Update example in Docs for captionElement

[DannyAziz]

The code example here (Switch between years and months) for captionElement doesn't take into consideration what month your toMonth or endMonth props fall on.

For example, if your forMonth prop falls on May of 2016 the component will still render Jan - Dec for 2016 but instead, you would only want it to show and start on May

Here's a code example for the desired functionality:

function YearMonthForm({ date, localeUtils, onChange }) {
    const months = localeUtils.getMonths();
    const monthsToRender = [];
    let currentYear = date.getFullYear();

    for (let month in months) {
        var dt = new Date(currentYear, month);
        var c = new Date(toMonth.getTime());   
        c.setDate(c.getDate()+1); 
        var inRange = dt >= fromMonth && dt < toMonth;
        if (inRange) {
            monthsToRender.push(months[month]);
        };
    };
  
    const years = [];
    for (let i = fromMonth.getFullYear(); i <= toMonth.getFullYear(); i += 1) {
      years.push(i);
    }
  
    const handleChange = function handleChange(e) {
      const { year, month } = e.target.form;
      onChange(new Date(year.value, month.value));
    };
  
    return (
      <form className="DayPicker-Caption">
        <select name="month" onChange={handleChange} value={date.getMonth()}>
          {monthsToRender.map((month, i) => (
            <option key={month} value={i}>
              {month}
            </option>
          ))}
        </select>
        <select name="year" onChange={handleChange} value={date.getFullYear()}>
          {years.map(year => (
            <option key={year} value={year}>
              {year}
            </option>
          ))}
        </select>
      </form>
    );
};