Add documentation for duals

[zickgraf]

There are two cases:

1. In the symmetric closed monoidal case, duals can be defined by an evaluation and a universal property and derived as hom(-,1).
2. In a symmetric monoidal category, duals can be defined via an evaluation and a coevalution. This sometimes is the definition of "rigid", but not in CAP.

The second case also defines a closed structure and then coincides with the first case. The corresponding derivations are already defined in CAP, the mathematics can be found here: https://mathoverflow.net/questions/141138/definitions-and-coherence-in-rigid-monoidal-categories

Issues which could also be handled in this context: #314 #319 #803