man: keyfile: improve explanation of the format and routes properties
Summary
Improve explanation of the keyfile format so it's easier to understand it.
Also complete the explanation about how to define routes and routing-rules.
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
-
the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable