chore(docs): add descriptive comments to structs and methods

## Description

Several exported structs, interfaces, and methods in the zaprpc package currently lack clear GoDoc-style comments.
This makes the codebase harder to navigate for contributors and users reading the source or using go doc.

This issue proposes adding concise, descriptive comments to all exported symbols in the repository — aligning with [Effective Go](https://golang.org/doc/effective_go#commentary)
and GoDoc best practices.

Improve code readability, maintainability, and auto-generated documentation by ensuring that every exported type and function has a proper GoDoc comment explaining what it does and how to use it.