Add `{desc}` format arg for pulseaudio `format-source`

[OJFord]

Presently {desc} is only a valid format specifier in format, where it refers to the sink's description; using it in format-source (intending to get the source's description) results in an error.

It would be nice if this could be used to show the names of both the active sink as well as the active source, especially if using them in two modules as described in #925 (comment).