docs(data-table): updating sort-direction docs (#706)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

src/components/data-table/table-header-cell.ts

@@ -1,7 +1,7 @@
 /**
  * @license
  *
- * Copyright IBM Corp. 2019, 2020
+ * Copyright IBM Corp. 2019, 2021
  *
  * This source code is licensed under the Apache-2.0 license found in the
  * LICENSE file in the root directory of this source tree.
@@ -95,7 +95,7 @@ class BXTableHeaderCell extends FocusMixin(LitElement) {
 
   /**
    * The table sort direction.
-   * If present, this table header cell will have a sorting UI.
+   * If present, this table header cell will have a sorting UI. Choose between `ascending` or `descending`.
    */
   @property({ reflect: true, attribute: 'sort-direction' })
   sortDirection?: TABLE_SORT_DIRECTION;