Update accessibility.mdx (#4362)

* Update accessibility.mdx

addresses a shortcoming in the current keyboard guidance on the a11y tab as discussed in #4342 (comment)

* chore: format

---------

Co-authored-by: Alison Joseph <[email protected]>

src/pages/components/dropdown/accessibility.mdx

@@ -51,9 +51,10 @@ each variant.
 
 #### Dropdown
 
-The default dropdown opens with `Enter`, `Space`, or `Down arrow`, with focus
-moving to the currently selected option. Arrow keys change options, and any
-option with focus is selected with `Space` or `Enter`, which also closes the
+The default dropdown opens with `Enter`, `Space`, or `Down arrow`. If an option
+is already selected or if `Down arrow` is used to open the component, the focus
+moves to the first or currently selected option. Arrow keys change options, and
+any option with focus is selected with `Space` or `Enter`, which also closes the
 dropdown. Pressing `Esc` or `Tab` closes a dropdown without changing the
 selected option.
 
@@ -72,11 +73,14 @@ selected option.
 
 #### Multiselect
 
-The multiselect opens with the `Enter` or `Space` keys, which are also used to
-toggle the selection of any option. Once items are selected, a tag appears in
-the field, showing a numerical representation of the number of items selected
-along with an ‘x’. Pressing `Esc` closes the multiselect. Pressing `Delete`
-while focus is in the collapsed field will clear all selected options.
+The multiselect opens with `Down arrow`, or with the `Enter` or `Space` keys,
+which are also used to toggle the selection of any option. Focus handling is the
+same as with the default dropdown: focus is only moved to the options list if an
+item is already selected or if the multiselect is opened with the `Down arrow`.
+Once items are selected, a tag appears in the field, showing a numerical
+representation of the number of items selected along with an ‘x’. Pressing `Esc`
+closes the multiselect. Pressing `Delete` while focus is in the collapsed field
+will clear all selected options.
 
 <Row>
 <Column colLg={8}>
@@ -93,15 +97,18 @@ while focus is in the collapsed field will clear all selected options.
 
 #### Combo box
 
-The combo box's operation is quite different, since it is a combination of a
-text input and a dropdown. `Enter` or the `Up` or `Down` arrow keys will open
-the list of options, as will typing any character or string of characters (which
-will also filter the list to only show items that match the typed string).
-Pressing `Esc` will clear the input and collapse the list. Pressing `Enter` will
-select a highlighted option and collapse the list. (If nothing is highlighted in
-the list, pressing `Enter` will toggle the combo box open and closed.) `Space`
-cannot be used for selecting, as pressing it will submit a space character into
-the filter string.
+The combo box’s operation (as well as the filterable multiselect's) is quite
+different, since it is a combination of a text input and a dropdown. `Enter` or
+the `Up` or `Down arrow` keys will open the list of options, as will typing any
+matching character or string of characters (which will also filter the list to
+only show items that match the typed string). Focus handling in the options list
+is similar to the default dropdown: focus is only moved to the options list if
+an item is already selected, a matching string is entered, or if opened with an
+arrow key. Pressing `Esc` will collapse the list or clear the typed string.
+Pressing `Enter` will select a highlighted option and collapse the list. (If
+nothing is highlighted in the list, pressing `Enter` will toggle the combo box
+open and closed.) `Space` cannot be used for selecting, as pressing it will
+submit a space character into the filter string.
 
 <Row>
 <Column colLg={8}>