docs/DESKTOP_ENVIRONMENTS: clarify name aliases

docs/DESKTOP_ENVIRONMENTS: clarify name aliases

Add a note to the service-file naming scheme that reminds developers
that those names might be aliases. Hence, when parsing such unit names,
the entire name-array of a unit must be parsed, rather than just the
unit ID.

The service-name of existing applications might be already part of their
API. Hence, not all applications can switch the service ID to this new
naming scheme, but can provide suitable aliases. Document this behavior.