docs/DESKTOP_ENVIRONMENT: clarify <RANDOM> usage

docs/DESKTOP_ENVIRONMENT: clarify <RANDOM> usage

The <RANDOM> part is optional in the naming scheme of application units.
However, this is only true for service files. Scope units must include
the <RANDOM> part, otherwise it would be impossible to parse:

The schema would be:

    `app[-<launcher>]-<ApplicationID>[-<RANDOM>].scope`

in which case a two-part name would be impossible to parse, since it is
unclear whether the launcher of the random bit where omitted.

(cherry picked from commit cbaebf811e6edb79279f72950b2d63189be314d3)