[a11y]: Dropdown component usage and style documentation problems

[mbgower]

This is a summary issue for various considerations in the dropdown documentation. The considerations are detailed in comments in a pdf export of each of the Usage and Style pages. They should be examined carefully.

In summary, the high level problems are:

• the 'placeholder' option that populates an unopened dropdown is not in the list of options. This is unusual (and is causing problems with implementation)
• Carbon should consider breaking combo box into its own component (as it is in storybook)
• Carbon should explore positives and negatives to making dropdown and select variants of the same component.
• there are a lot of passages in the Usage text that need to be revisited
• consider only using Space to select/deselect checkboxes in the multiselect
• consider making a single option selected in a multiselect appear in the field (styled as a tag, with an X if necessary; although one could argue that is unnecesary and it could just be a text value)
• focus and highlighting on the options list and options within the list is problematic (detailed in Style page)

Note: this issue on documentation is additional to the following issues opened against the implementation of dropdown and its variants:

• [a11y]: Dropdown component contrast for item with focus/selection fails carbon#12720
• [a11y]: Dropdown and multi-select dropdown inconsistent and have implementation issues carbon#12721
• [a11y]: Dropdown does not use aria-describedby to link help text with component carbon#12722
• [a11y]: Various Multi-select dropdown problems carbon#12723

[sstrubberg]

Closing in favor of tracking via carbon-design-system/carbon#13521