[backport#12452] docs: clarified systemd installation instructions in init.md for Ubuntu users.

Summary:
4d14d06fc docs: clarified systemd installation instructions in init.md for Ubuntu users. (DaveFromBinary)

Pull request description:

Added a note to init.md to clarify the .service copy path for Ubuntu because it differs from the described copy path.

Also noted which version of Ubuntu switched to systemd for the default system init to clarify when the systemd installation steps should be used instead of the upstart installation steps for Ubuntu users.

Backport of Core PR12452

Test Plan: read it

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8207