Update alt texts to describe what the screenshot is showing

[bencomp]

The few screenshots in the lesson have alt texts that are not descriptive; they barely mention the name of the interface element.

The alt text should briefly explain what is important about the screenshot, instead of their current values:

• episode 2: "Parse options"
• episode 2: "OR Transform"
• episode 5: "History"

[bencomp]

As discussed in the 2025-07-16 Maintainers meeting, see https://www.stylemanual.gov.au/content-types/images/alt-text-captions-and-titles-images for explanations and examples.