man: Add documentation for generator path environment variables

man: Add documentation for generator path environment variables

This adds documentation for the SYSTEMD_GENERATOR_PATH and
SYSTEMD_ENVIRONMENT_GENERATOR_PATH variables to the systemd man page
grouped with the existing SYSTEMD_UNIT_PATH.

Also added is a description about how these variables work, i.e. that a
trailing : can be used to prepend paths to the usual set.