The Apple computer operating system was renamed from "OS X" to "macOS"
in 2016. We should accept "macos" instead of "osx" in all our CLI APIs,
and the docs should also mention this alias instead of "osx".
For backwards compatibility and for compatibility with the tldr client
specification, we should still accept "osx" though.
This makes it compliant with the client spec.
The `-p` flag was previously used for the pager, using the pager now
requires using the long flag (`--pager`). Alternatively the pager can be
configured through the config file.
This commit also renames OsType to PlatformType.