`@Description` annotation

[lars-reimann]

Is your feature request related to a problem? Please describe

It's not possible at the moment to update the description of an API element.

Desired solution

@Description annotation that is

• applicable to all API elements
• takes the new description as input
• can be used at most once per element.

[lars-reimann]

The branch 517-description-annotation already contains the required changes to the backend.