Refresh expand() docs

[batpigandme]

• Proofread title, description, and parameters
• Proofread comments in examples
• Ensure comment headings used to divide into scannable sections
• Flag examples that seem overly complex for further review
• Switch out mtcars/iris for something more interesting, or a smaller dataset constructed specifically to illustrate one point.
• Review size of example outputs to ensure that you can easily see important rows and cols.
• Datasets should always be tibbles to avoid use of as_tibble() distracting from the main point