docs: Polish docs for write with if not exists

[Xuanwo]

Which issue does this PR close?

None

Rationale for this change

Improve docs

What changes are included in this PR?

Make the behavior of different options more clear.

Are there any user-facing changes?

No, just docs change.

[codspeed-hq]

CodSpeed Performance Report

Merging #5320 will degrade performances by 18.88%

_{Comparing polish-if-not-exsts (98528e9) with main (cd2fc9f)}

Summary

❌ 8 regressions
✅ 65 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	polish-if-not-exsts	Change
❌	buffer 256 KiB * 32 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 256 KiB * 4 chunk	125.3 ns	154.4 ns	-18.88%
❌	buffer 4.00 MiB * 32 chunk	125.3 ns	154.4 ns	-18.88%
❌	buffer 4.00 MiB * 4 chunk	125.3 ns	154.4 ns	-18.88%
❌	256 KiB * 1000k truncate	245.8 ns	275 ns	-10.61%
❌	256 KiB * 100k truncate	245.8 ns	275 ns	-10.61%
❌	256 KiB * 10k truncate	245.8 ns	275 ns	-10.61%
❌	256 KiB * 1k truncate	245.8 ns	275 ns	-10.61%