Add documentation on pivots

[BentleyJOakes]

The documentation does not describe pivots and their semantics.

From what I can tell, a) the pivot name maps to one element at a time, b) only the LHS matches or sets a pivot, and the NAC can match on pivots (so the RHS has no interaction with pivots), and c) the pivot will not be set if the NAC matches.

This information should be added to the documentation and extended with examples such as the Petri Net formalism.