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.