MenuToggle - Update a11y docs to include mention dynamic toggle label + aria attributes

[thatblindgeye]

Per patternfly/patternfly#6562 (comment), we should update MenuToggle a11y docs to mention that when the toggle content is expected or intended to be dynamic (such as when used for a Select, where a selected option might become the toggle content), there should also be a static label for the toggle.

Additionally, we should mention when aria-invalid="true" and aria-autocomplete="list" should be applied (the former whenever status is danger, the latter for typeahead toggles only)