Somewhat clarified how constraints are stored on a model, and how their
initial values are set:

* Constraints are treated in two separate categories: 'parameter
  constraints' which are constraints on specific parameters, and
  'model constraints' which are general constraints on the model
  (specifically the 'eqcons' and 'ineqcons' constraints).
* All constraints are stored in the ._constraints dict.
* Setting of initial values of constraints is simplified so that only
  Model._initialize_constraints is responsible for this (though it still
  queries the individual parameters for any default constraints they may
  have).
Docstring tweaks as suggested by @cdeil