Improve documentation about non-executed rules

[redeboer]

It's not clear what has happened when the STM found results, but that no rules were executed. This has to be better documented, for instance, by providing docstrings for ExecutionInfo.not_executed_node_rules.

Example

pip install qrules==0.9.7

from qrules.transition import InteractionType, StateTransitionManager

stm = StateTransitionManager(
    initial_state=["J/psi(1S)"],
    final_state=["K0", "Sigma+", "p~"],
    allowed_intermediate_particles=["Delta(1910)+"],
    formalism="canonical",
)
stm.set_allowed_interaction_types([InteractionType.STRONG])
problem_sets = stm.create_problem_sets()
reaction = stm.find_solutions(problem_sets)

RuntimeWarning: There are conservation rules that were not executed:
gellmann_nishijima, isospin_conservation, parity_conservation,
isospin_validity, spin_magnitude_conservation, StrangenessConservation,
MassConservation, BaryonNumberConservation, ChargeConservation,
spin_validity, identical_particle_symmetrization, helicity_conservation,
g_parity_conservation, parity_conservation_helicity, c_parity_conservation