Apple's desktop operating system was formerly called "Mac OS X" and "OS X", but since 2016 it has been called "macOS" (starting with version 10.12).
Changing across all comments and documentation to use this current terminology, except in cases where the historical versions are explicitly referenced. No code changes are made; we should consider changing those in future PRs.
The file uses a mixture of UTF-8 and CP-1252, which libiconv doesn't like. This changes it to be entirely UTF-8.
Also fixed a broken URL and corrected the typography slightly.
