doc: update os.markdown
Some doc update based on improvement ideas I remember from when I used this module in node:
- Mention windows returns 0 for
nice
values (which is obvious, but io makes no attempt to calculate something similar or return undefined, 0 is returned) - Mention platform and arch are aliases for
process
properties. - Document possible return values where appropriate, add examples in others.
- Rename title in order to match other titles in the navigation.
- Fix line that was over 80 characters long.