update the documentation

[johnyf]

As of a808dc1a5b02d451ddce6e33640562f00a2a4771,doc.md is about to start becoming outdated:

• The viewpoint has now shifted to untyped logic: describe role of refinement mappings.
• First-order logic is directly available in omega.symbolic.fol: explain how to use it.
• Describe enumeration of symbolic entities.
• The cumbersome unidirectional two-stage building of omega.symbolic.symbolic.Automaton will be replaced, by subclassing from fol.Context.
• De-emphasize enumerated "transition systems" and "automata".
• Demonstrate Streett and Rabin solvers via examples (strategies, counter-strategies).
• Mini howto for fixpoint computations.

[johnyf]

Addressed in 3c492a6a326eede360e51f51d7914489cd39f87c.