fix: memory leak when positioning with popper (#1094)

* fix: memory leak when positioning with popper
* docs: update calculate position docs

Co-authored-by: Jeff <sagalbot@gmail.com>

docs/api/props.md

@@ -128,6 +128,9 @@ transition: {
 
 When `appendToBody` is true, this function is responsible for positioning the drop down list.
 
+If a function is returned from `calculatePosition`, it will be called when the drop down list
+is removed from the DOM. This allows for any garbage collection you may need to do.
+
 See [Dropdown Position](../guide/positioning.md) for more details.
 
 ```js
@@ -139,6 +142,7 @@ calculatePosition: {
      * @param width {string} calculated width in pixels of the dropdown menu
      * @param top {string} absolute position top value in pixels relative to the document
      * @param left {string} absolute position left value in pixels relative to the document
+     * @return {function|void}
      */
     default(dropdownList, component, {width, top, left}) {
       dropdownList.style.top = top;