man: rework the description of Aliases and .wants/.requires directories

The description of Alias= wasn't incorrect, but it sounded like Alias= creates
a different type of dependency, while it's just a glorified way to create
symlinks. Also recommend 'preset' in addition to 'enable'.

Describe .wants/.requires dirs as equals, without implying that the [Install]
section can only be used for .wants.

The text was partially out of date (systemd-networkd.service now creates as
alias in /etc, not /usr/lib, let's just not say anything about the full path).

(cherry picked from commit b5328434)