update description of the 3 types of dropdown

What does this MR do?

Update the description of dropdown and its usage:

• Old: two types of dropdown: plain and checkmark
• New: three types of dropdown: plain, single-selection and multi-selection

Does this MR meet the acceptance criteria?

• Add a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Include MR scope that reflects the purpose of the change in the description
  • For example, if this MR is focused on usage guidelines, addressing accessibility challenges could be added separately with a different MR
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