Some small improvements

Small improvements of the operational action section and the common sliver info return :error section.
This removes some TODOs in the text.


Note on minor API change:
The :error explanation was shortened to make it more simple. This removed some requirements that were in AMv3. In short, these requirements were: 
* :error is always present in a Status return
* :error is never present in an Allocate return
I don't think these requirements were very useful and only complicate things.