Improve documentation of dihedrals

[hmcezar]

Currently, in the Theory part of the documentation, there's no mention of different types of dihedrals or the meaning of the [-1], or the [1.0] in the lists.

However in the API part the dataclass seems to be compatible with different types of dihedral. On top of that, the expression for V_4 is different in the "Theory" and the "API".