[Divider] Update flexItem description

docs/pages/api/divider.md

@@ -27,7 +27,7 @@ You can learn more about the difference by [reading this guide](/guides/minimizi
 | <span class="prop-name">absolute</span> | <span class="prop-type">bool</span> | <span class="prop-default">false</span> | Absolutely position the element. |
 | <span class="prop-name">classes</span> | <span class="prop-type">object</span> |  | Override or extend the styles applied to the component. See [CSS API](#css) below for more details. |
 | <span class="prop-name">component</span> | <span class="prop-type">elementType</span> | <span class="prop-default">'hr'</span> | The component used for the root node. Either a string to use a DOM element or a component. |
-| <span class="prop-name">flexItem</span> | <span class="prop-type">bool</span> | <span class="prop-default">false</span> | If `true`, the divider will adapt to a parent flex container. |
+| <span class="prop-name">flexItem</span> | <span class="prop-type">bool</span> | <span class="prop-default">false</span> | If `true`, a vertical divider will have the correct height when used in flex container. (By default, a vertical divider will have a calculated height of `0px` if it is the child of a flex container.) |
 | <span class="prop-name">light</span> | <span class="prop-type">bool</span> | <span class="prop-default">false</span> | If `true`, the divider will have a lighter color. |
 | <span class="prop-name">orientation</span> | <span class="prop-type">'horizontal'<br>&#124;&nbsp;'vertical'</span> | <span class="prop-default">'horizontal'</span> | The divider orientation. |
 | <span class="prop-name">variant</span> | <span class="prop-type">'fullWidth'<br>&#124;&nbsp;'inset'<br>&#124;&nbsp;'middle'</span> | <span class="prop-default">'fullWidth'</span> | The variant to use. |

packages/material-ui/src/Divider/Divider.js

@@ -99,7 +99,8 @@ Divider.propTypes = {
    */
   component: PropTypes.elementType,
   /**
-   * If `true`, the divider will adapt to a parent flex container.
+   * If `true`, a vertical divider will have the correct height when used in flex container.
+   * (By default, a vertical divider will have a calculated height of `0px` if it is the child of a flex container.)
    */
   flexItem: PropTypes.bool,
   /**