While reviewing #9942 I noticed that the documentation on the netdev
driver API is unclear and in some cases outright contradicting itself:

> ```
> @return              number of bytes used from @p value
> @return              `< 0` on error, 0 on success
> ```

IMHO this is unacceptable for such a central API where communication

This fixes a few things and also clarifies preconditions:

- Specifies negative `errno`s clearly so all drivers return the same
  when required
- Re-iterates parameter preconditions and special cases within the
  parameter documentation itself (might also help towards #9805?)
- Fixes contradictions within return value documentation
- Adds missing parameter documentation on `init()`.