docs: improve documentation formatting, punctuation, and spelling

[mevinagrise]

This PR improves the documentation comments in src/lib.rs to better align with Rust documentation standards (RFC 1574) and the project's coding guidelines.

Specific changes include:

• Punctuation & Capitalization: Added missing periods at the end of single-line doc comments for methods like insert, remove, any, and test to ensure they form proper summary sentences. Capitalized the first letter of the bitset field documentation.
• Spelling: Corrected a typo in the bitset doc comment (inavailable -> unavailable).
• Comment Clarity: Improved an inline comment in dealloc_contiguous. Changed the descriptive // Check if the range is valid. to an intent-revealing comment // Prevent out-of-bounds access by ensuring the deallocation range fits within capacity., explaining the why rather than just the what.

Related Guidelines:

• comment-punctuation: Ensure all full-sentence comments end with a period.
• explain-why: Replace comments that describe what the code does with comments explaining the intent or reasoning.

Testing:

• Ran cargo doc --no-deps to ensure documentation renders correctly.
• Ran cargo test to ensure no code logic was affected.

[Copilot]

Pull request overview

This PR polishes Rust documentation comments in src/lib.rs to better align with Rust doc comment conventions (sentence-style summaries, punctuation, spelling) and improve inline comment clarity.

Changes:

• Added missing punctuation/capitalization to several doc comments.
• Fixed a spelling typo in a doc comment.
• Reworded an inline comment in dealloc_contiguous to explain intent.

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[mevinagrise]

I’ve updated the code based on Copilot’s feedback.