Improving enum documentation

[erikbosch]

Fixing two flaws in enum documentation

1. Enums can (and are) also used for attributes, not only sensors and actuators
2. Enums can be used for both strings and ints.
   Current example only showed int-example, while VSS so far just use
   strings.

Also moving down enum to after attributes

[erikbosch]

Discussed at meeting 2021-08-10. Remove old example with int, as we do not use int enums in VSS. To be further discussed in subsequent meetings

[erikbosch]

In the meeting 2021-08-10 it was discussed if supporting integer-types with enums makes sense. We do not actively use it today in VSS, but I could see a possible use for it in some cases, like when representing OBD-signals where enums are defined by numbers rather than by name (like https://en.wikipedia.org/wiki/OBD-II_PIDs#Service_01_PID_1C). By specifying exactly which values that are allowed a VSS-server could theoretically log an error if someone reports the value 14 (reserved) for PID 1C. If we only specify datatype (uint8) or min/max (0-250) this will not be detected

[SebastianSchildt]

Meeting 08/28 merge this, and keep discussing the enum issue