Add description for volume applet

Summary:
Fixes appstream test failure on CI.
Also it's a useful thing to have anyway as the comment is shown in the
Widget Explorer.

This breaks the string freeze, but it is a bug fix.
CC-MAIL: kde-i18n-doc@kde.org

Test Plan: appstreamcli validate now passes

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2829