docs: Polish docs for Operator, Reader and Writer

[Xuanwo]

Which issue does this PR close?

None.

Rationale for this change

Make user happy about our public API.

What changes are included in this PR?

Docs for all options provided by opendal Reader and Writer.

Are there any user-facing changes?

API Docs

[codspeed-hq]

CodSpeed Performance Report

Merging #5516 will degrade performances by 15.89%

_{Comparing polish-docs (c3bb036) with main (6ca3eab)}

Summary

⚡ 8 improvements
❌ 4 regressions
✅ 61 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	polish-docs	Change
❌	buffer 256 KiB * 32 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 256 KiB * 4 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 4.00 MiB * 32 chunk	154.4 ns	183.6 ns	-15.89%
❌	buffer 4.00 MiB * 4 chunk	154.4 ns	183.6 ns	-15.89%
⚡	256 KiB * 1000k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 1000k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 100k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 100k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 10k chunk	183.6 ns	154.4 ns	+18.88%
⚡	256 KiB * 10k truncate	245.8 ns	216.7 ns	+13.46%
⚡	256 KiB * 1k chunk	212.8 ns	183.6 ns	+15.89%
⚡	256 KiB * 1k truncate	245.8 ns	216.7 ns	+13.46%