Doc generator subset traversal needs to follow "where used" rather than "where defined" approach

[akf]

Take for example this path in a subset doc:

   "Processor": {
      "Properties": {
         "OperatingSpeedRangeMHz": {
            "Properties": {
               "ControlMode": {
                  "SupportedValues": [
                     "Automatic",
                     "Disabled",
                     "Manual",
                     "Override"
                  ]
               }
            }
         }
      }

OperatingSpeedRangeMHz is defined as a reference to a property definition in the Control schema. When the doc generator looks for a subset definition for the ControlMode property, it's already working within the context of the Control schema, so it doesn't find this subset definition.

Authors might need to define different subsets for the "same" property in the context of different schemas, so the fix is not to change the subset doc. We need the doc generator to look up the subset in the context where the property is used.