Document in great detail where constraints for data leafs is authoritatively defined

Problem/Motivation

Found in #3456008: [later phase] Support matching enum SDC prop shapes against DynamicPropSources, not only generating StaticPropSources
All field item constraints are not found when evaluating prop matches. There are couple problem with this.

1. $complex_data_constraint = array_filter(
         $field_item->getConstraints(),
         fn ($c) => $c instanceof ComplexDataConstraint
       );
       if (!empty($complex_data_constraint)) {
         $field_item_level_constraints_indirect = reset($complex_data_constraint)
           ->properties[$field_property_name] ?? [];
       }
       else {
         $field_item_level_constraints_indirect = [];
       }
   

   $complex_data_constraint is an array but we only check the first element in the array. I have worked on temporary fix in #3456008 that I will bring in here and it does find more constraints

2. $rekey = function (array $constraints) {
         return array_combine(
           array_map(
             fn (Constraint $c): string => get_class($c),
             $constraints,
           ),
           $constraints
         );
       };
   

   This is assumes that classes among multiple constraints will be unique but this is not necessarily the case. I haven't checked if we have any cases of this yet though.

3. $field_item_level_constraints_direct = $field_item->getConstraints()[$field_property_name] ?? [];
   

   I think the array returned from getConstraints() is actually always a list so this will never have a key of the property name. In #3456008 I did through an exception if this was to ever happen and it was never hit in our tests.

Steps to reproduce

Proposed resolution

Fix the bugs
Update the test expectations and see if they are reasonable

Remaining tasks

User interface changes

API changes

Data model changes