Checkbox: add jsdoc comment

app/soapbox/components/ui/checkbox/checkbox.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 
 interface ICheckbox extends Pick<React.InputHTMLAttributes<HTMLInputElement>, 'disabled' | 'id' | 'name' | 'onChange' | 'checked' | 'required'> { }
 
+/** A pretty checkbox input. */
 const Checkbox = React.forwardRef<HTMLInputElement, ICheckbox>((props, ref) => {
   return (
     <input