man: fix invalid description of template handling in WantedBy=

man: fix invalid description of template handling in WantedBy=

We don't need to talk about Alias=. The approach of using Alias= to enable
units is still supported, but hasn't been advertised as the way to do thing
for many years. Using it as an explanation is just confusing.

Also, the description of templated units did not take DefaultInstance=
into account. It is updated and extended.