Modal, Dropdown add dispose method to our documentation

Created by: Johann-S

In our docs : https://getbootstrap.com/docs/4.0/components/modal/#methods https://getbootstrap.com/docs/4.0/components/dropdowns/#methods

we cannot find dispose methods but it's a public method in our code see : https://github.com/twbs/bootstrap/blob/v4-dev/js/src/modal.js#L208 https://github.com/twbs/bootstrap/blob/v4-dev/js/src/dropdown.js#L175

Related to : #24045 (closed)