Doxygen eliminate warnings pt. 2

This PR eliminates all remaining doxygen warnings that are no "not documented" ones.
Bad english, my proposal:
```
pointer to an external task handler that is executed, if an external interrupt occurrs and the external subroutine is left.
This is an optional parameter therefore NULL is a legal value.
```
I have to admit though, that I don't really understand what the "external subroutine is leaved" is about in the first place. If that's important I think it should be fleshed out a bit, otherwise I guess you could drop it alltogether.