make `netwatcher`'s api more useful, better documented

[divagant-martian]

netwatcher has a very small API that might not be as useful to the general public. It only gives a subscribe to changes that only reports if the change "is major" without any description of what being major is or the origin of the change. It would be good Providing a more detailed API and better docs.

Some inspiration: crates.io/crates/netwatcher