man: drop recommendation to use absolute paths in ExecStart*=

man: drop recommendation to use absolute paths in ExecStart*=

The only reason to recommend this would be if people had multiple commands
with the same name in the search path. This probably was never the best idea,
and it happens rarely anyway. Since the patch that dropped requirement for full
paths was introduced, we have dropped support for unmerged-usr and we're planning
to drop support for split-bin at some point too. Many people effectively have just
one directory in the search path, so there is even less reason to use an absolute
path. So let's recommend just using the command name, which makes the unit file
shorter and nicer to read.