docs: document using as_borrowed in the Bound migration

[LilyFirefly]

As suggested in #3826 (comment).

[codspeed-hq]

CodSpeed Performance Report

Merging #3833 will degrade performances by 16.95%

_{Comparing LilyFoote:document-as-borrowed (097f32c) with main (e308c8d)}

Summary

⚡ 1 improvements
❌ 3 regressions
✅ 75 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	LilyFoote:document-as-borrowed	Change
❌	mapping_from_dict	272.2 ns	327.8 ns	-16.95%
⚡	f64_from_pyobject	433.3 ns	377.8 ns	+14.71%
❌	sequence_from_list	272.2 ns	327.8 ns	-16.95%
❌	sequence_from_tuple	232.8 ns	260.6 ns	-10.66%

[davidhewitt]

Thanks!