Docs: Make tone more factual

[starkmarkus]

Summary

Small documentation pass to replace a few first-person and conversational phrases with factual wording in the ML-KEM and HKDF crate docs/comments.

Issue

Relates to: #26

Scope

• Reword a handful of crate-doc sentences in crypto/mlkem/src/lib.rs
• Reword a handful of crate-doc comments in crypto/hkdf/src/lib.rs

Validation

• git diff --check