Cleanup: Anim, add some more comments to the bone collection system

Add some TODOs and clarification in comments.

No functional changes.

source/blender/animrig/intern/bone_collections.cc

@@ -371,6 +371,8 @@ bool ANIM_armature_bonecoll_unassign(BoneCollection *bcoll, Bone *bone)
 void ANIM_armature_bonecoll_unassign_all(Bone *bone)
 {
   LISTBASE_FOREACH_MUTABLE (BoneCollectionReference *, ref, &bone->runtime.collections) {
+    /* TODO: include Armature as parameter, and check that the bone collection to unassign from is
+     * actually editable. */
     ANIM_armature_bonecoll_unassign(ref->bcoll, bone);
   }
 }

source/blender/editors/armature/editarmature_undo.cc

@@ -100,6 +100,8 @@ static void undoarm_to_editarm(UndoArmature *uarm, bArmature *arm)
   auto bcoll_map = ANIM_bonecoll_listbase_copy_no_membership(
       &arm->collections, &uarm->bone_collections, true);
 
+  /* Always do a lookup-by-name and assignment. Even when the name of the active collection is
+   * still the same, the order may have changed and thus the index needs to be updated. */
   BoneCollection *active_bcoll = ANIM_armature_bonecoll_get_by_name(arm,
                                                                     uarm->active_collection_name);
   ANIM_armature_bonecoll_active_set(arm, active_bcoll);

source/blender/makesrna/intern/rna_armature.cc

@@ -1834,6 +1834,8 @@ static void rna_def_armature_collections(BlenderRNA *brna, PropertyRNA *cprop)
 
   prop = RNA_def_property(srna, "active_name", PROP_STRING, PROP_NONE);
   RNA_def_property_string_sdna(prop, nullptr, "active_collection_name");
+  /* TODO: For some reason the overrides system doesn't register a new operation when this property
+   * changes. Needs further investigation to figure out why & fix it. */
   RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_LIBRARY);
   RNA_def_property_ui_text(prop,
                            "Active Collection Name",