command: format ${avsync} like on terminal status line

Padding with spaces is very useless for OSD (because most fonts are
variable width), but it's good when using it on the terminal, e.g. for
reproducing the default terminal status line.
manpage: ao_alsa: link upmix wiki entry

Also clarify the statement about what we expect to happen by default.
It's well possible that distros at some point will fix their ALSA
configuration, and e.g. enable the upmix plugin by default.