more informative error message on nested dont statements in roxygen code examples

[lorenzwalthert]

Addresses #498.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 269ec40 is merged into main:

• ✔️cache_applying: 27.7ms -> 28ms [-0.81%, +2.75%]
• ✔️cache_recording: 395ms -> 394ms [-0.89%, +0.33%]
• ✔️without_cache: 928ms -> 923ms [-1.01%, 0%]

Further explanation regarding interpretation and methodology can be found in the documentation.