The reason for depending on the socket unit is rather unobvious, so
let's add a comment to help people reading the service unit file. Felipe
Sateler explained the rationale well in the commit message of
7cb524a77b, so I just copied the same text into the comment.
This commit fixes two problems:
1. Because there are no implicit dependencies between sockets and services,
the socket, as set up by systemd will race with the socket, as set up
by the pulseaudio daemon. This can cause the pulseaudio.socket unit to
fail (even though the pulseaudio service started just fine), which can
confuse users.
2. While it is possible to use the service without the socket, it is not
clear why it would be desirable. And a user installing pulseaudio and
doing `systemctl --user start pulseaudio` will not get the socket
started, which might be confusing and problematic if the server is to
be restarted later on, as the client autospawn feature might kick in.
