List iframe elements as a limitation in prompt (#6309)

Author: jennifer-shehane

docs/api/commands/prompt.mdx

@@ -725,7 +725,7 @@ cy.prompt(electronicsFilterPrompt)
 | Browser Support  | Chromium-based browsers only (Chrome, Edge, Electron)                    |
 | Language Support | Optimized for English prompts; other languages are not guaranteed        |
 | Assertions       | 'not.exist' assertions are not supported                                 |
-| DOM Support      | Canvas elements are not supported                                        |
+| DOM Support      | Canvas and iframe elements are not supported                             |
 | Command Coverage | Not all Cypress APIs supported - see [What you can do](#What-you-can-do) |
 | Step Limit       | Each `cy.prompt` call is limited to 50 steps                             |