Update InputDropdown docs for clarity

[clarsen-czi]

The documentation for InputDropdown isn't entirely clear for how to use Label vs Value variants.

Docs currently say: "Before a selection has been made, the input should display a prompt instructing users to make a selection" which isn't entirely true since the input doesn't need to have a propt so much as the label for what elements are to be found in the Dropdown.

Nina from Eng recommends describing the differences in terms of the Value variant requires a separate HTML element for the the label whereas the Label variant does not and incorporates the label into the input.