[Doc] Update doc of custom logit processor

[hongpeng-guo]

Motivation

This is a follow-up PR of #2396 that updates the sampling_params.md with better doc string and examples.

Modifications

1. Updated relevant fields in sampling_params.md
2. Updated relevant fields io_struct.py to improve the doc string.

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling.