docs: standardize public API documentation style

[tisonkun]

Summary

• Standardize publicly reachable rustdoc summaries on third-person present tense, using forms such as Creates, Updates, and Returns.
• Normalize sentence punctuation, inline-code formatting for identifiers and ranges, and lg_k terminology while retaining exact Rust parameter names where needed.
• Reorder existing sections as Examples, Errors, and Panics, followed by compatibility notes; no failure-semantics sections were added.
• Document these conventions in CONTRIBUTING.md.

Scope

This is a documentation-only refactor. It does not change code, public APIs, or runtime behavior.

Validation

• cargo x check
• cargo x test
• cargo x lint
• Rustdoc JSON audit across all features for summary voice, punctuation, terminology, and section order

[tisonkun]

All reviewed. The documents of HllType can be reverted to contain more details. But the current version focuses more on user-facing information and I think it's fair enough to do the change now until someone has a different opinion.

Merging ...