Draft: add some explanation on the properties of PathArray

The use of `App::PropertyLinkSubList` for `'PathSubelements'`
is a mistake because we could use a single `App::PropertyLinkSub`
to handle both `'PathObject'` and `'PathSubelements'` properties.

This commit doesn't change any code, it just adds comments
explaining the situation so that it is not forgotten,
and we remember to address it in the future.

Ideally we should migrate the objects, but we may also decide
to break compatibility with older `PathArrays` if both properties
can't be migrated easily.

src/Mod/Draft/draftobjects/patharray.py

@@ -28,7 +28,36 @@
 """Provides the object code for the Draft PathArray object.
 
 The copies will be placed along a path like a polyline, spline, or bezier
-curve.
+curve, and along the selected subelements.
+
+To Do
+-----
+The `'PathSubelements'` property must be changed in type, as it does not need
+to be an `App::PropertyLinkSubList`.
+A `LinkSubList` is to select multiple subelements (edges) from multiple
+objects. However, since we need to select a `'Path Object'` already,
+which is a single object, the subelements that we can choose must belong
+to this `'Path Object'` only.
+
+Therefore, the correct property that must be used is `App::PropertyLinkSub`.
+Then in the property editor we will be unable to select more than one object
+thus preventing errors of the subelements not matching the `'PathObject'`.
+
+In fact, both `'PathObject'` and `'PathSubelements'`
+could be handled with a single `App::PropertyLinkSub` property,
+as this property can be used to select a single object,
+or a single object with its subelements.
+
+In the future, we could migrate the properties, or outright break
+compatibility with older objects by changing both properties
+`'PathObject'` and `'PathSubelements'`.
+
+An alternative to this would be to use a single `App::PropertyLinkSubList`.
+This would allow us to build PathArrays on multiple objects and multiple
+subelements (edges) of those objects at the same time. However, to do this,
+the logic in `execute` would have to be changed to account for multiple
+objects. Therefore, the first solution is simpler, that is, using
+a single property of type `App::PropertyLinkSub`.
 """
 ## @package patharray
 # \ingroup DRAFT
@@ -149,6 +178,13 @@ def set_general_properties(self, obj, properties):
                             _tip)
             obj.PathObject = None
 
+        # TODO: the 'PathSubelements' property must be changed,
+        # as it does not need to be an 'App::PropertyLinkSubList'.
+        #
+        # In fact, both 'PathObject' and 'PathSubelements'
+        # could be handled with a single 'App::PropertyLinkSub' property,
+        # as this property can be used to select a single object,
+        # or a single object with its subelements.
         if "PathSubelements" not in properties:
             _tip = _tr("List of connected edges in the 'Path Object'.\n"
                        "If these are present, the copies will be created "