docs: update example for storing Py<T> in structs

[davidhewitt]

As per #3775 (review)

There's a lot of documentation around the place which I'm sure needs updating; I'm hoping to take a pass at the guide later this week. For now, this is hopefully a start...

[codspeed-hq]

CodSpeed Performance Report

Merging #3876 will degrade performances by 15.02%

_{Comparing davidhewitt:update-example (769846e) with main (0bb9cab)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 77 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	davidhewitt:update-example	Change
❌	list_via_downcast	157.2 ns	185 ns	-15.02%
⚡	sequence_from_list	355.6 ns	300 ns	+18.52%