doc: make buffer.md more accurate

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

It may be easier to review the changes commit by commit, where they are grouped by a category. Last two commits are the biggest, but also easily skimmable.

• Remove duplication in a description.
• Fix possible typo.
• Add missing types.
• Unify number/integer types.
• Make notes about TypeError conditions in Buffer.from() variants more accurate.
• Add a general note about integer coercion for Buffer octets.
• Mark optional parameters as optional.
• Add notes about negative offsets.
• Unify periods in comments (except "Prints: ..." comments to avoid confusion).
• Unify link formatting.