[Docs] Better communicate when it's appropriate to use `SearchField` component variants

[brian-smith-tcril]

https://paragon-openedx.netlify.app/components/searchfield/

The top example on the docs site shows

In this screenshot:

• The user has typed "test"
• The 🔍 icon has disappeared
• The ❌ icon is the only thing that displays
• No search has happened (only onSubmit is wired up, nothing has been submitted)

This example of the component would be fine with search-on-type, but leaving the user in a state of having typed in a query, not searched using the query, and only seeing the ❌ icon isn't reasonable