docs(combobox, combobox-item): update token descriptions (#11806)

**Related Issue:** #7180

## Summary
Tweaks the token documentation descriptions for the `combobox` and
`combobox-item` components.

Author: geospatialem

Diff for: packages/calcite-components/src/components/combobox-item/combobox-item.scss

@@ -3,8 +3,8 @@
  *
  * These properties can be overridden using the component's tag as selector.
  *
- * @prop --calcite-combobox-text-color: Specifies the component's text color.
- * @prop --calcite-combobox-text-color-hover: Specifies the component's text color when hovered.
+ * @prop --calcite-combobox-text-color: Specifies the component's text and `icon` color.
+ * @prop --calcite-combobox-text-color-hover: Specifies the component's text and `icon` color when hovered.
  * @prop --calcite-combobox-item-background-color-active: Specifies the component's background color when active.
  * @prop --calcite-combobox-item-background-color-hover: Specifies the component's background color when hovered.
  * @prop --calcite-combobox-selected-icon-color: Specifies the component's selected indicator icon color.

Diff for: packages/calcite-components/src/components/combobox/combobox.scss

@@ -9,7 +9,7 @@
  * @prop --calcite-combobox-input-border-color: Specifies the border color of the component's input.
  * @prop --calcite-combobox-input-background-color: Specifies the background color of the component's input.
  * @prop --calcite-combobox-input-height: Specifies the height of the component's input.
- * @prop --calcite-combobox-input-text-color: Specifies the text color of the component's input.
+ * @prop --calcite-combobox-input-text-color: When `selectionDisplay` is `"single"`, specifies the text color of the component's input.
  */
 
 :host {