The dh_installsystemd.1 man-page does not describe the --no-also parameter.

File dh_installsystemd has this comment:

"Handle all unit files specified via Also= explicitly. This
is not necessary for enabling, but for disabling, as we
cannot read the unit file when disabling as it has already
been deleted. The undocumented --no-also option disables
handling of units linked via Also=. This option is provided
only to support a very specific use case in network-manager."

This reasoning does not hold true. Other packages like libvirt are using
the same parameter. To make developer's life easier the parameter should
be documented.

Best regards

Heinrich