ethclient: improve documentation comments

[Tinu280]

This PR improves several documentation comments in the ethclient package.

Changes include:

• More descriptive and consistent function-level comments
• Clarification of behavior and return values for common RPC wrapper methods
• Formatting fixes to align with Effective Go and gofmt/goimports guidelines
• No functional or logic changes

These updates aim to make the ethclient API clearer and easier to understand for new contributors and users.

Please let me know if further adjustments are needed.

[Tinu280]

Hi maintainers 👋

This PR updates documentation comments in the ethclient package only.
No logic or behavior changes are included.

All comments were formatted using gofmt/goimports and aligned with Effective Go documentation style.

If any additional adjustments are needed, I'm happy to make them.

Thanks for your time and review.