Cleanup: Anim, add explanation to bone collection assign operator

Just a little explanation of the code, because I got confused by my own.

No functional changes, except for the addition of a `BLI_assert_msg()`.

source/blender/editors/armature/bone_collections.cc

@@ -417,7 +417,10 @@ static int bone_collection_assign_exec(bContext *C, wmOperator *op)
     return OPERATOR_CANCELLED;
   }
 
+  /* NOTE: this operator can be called through the M/Shift+M shortcuts, which
+   * allow assigning to a newly-created bone collection. */
   BoneCollection *bcoll = get_bonecoll_named_or_active(C, op, ob, CREATE_IF_MISSING);
+  BLI_assert_msg(bcoll, "Bone Collection should always be created");
   if (bcoll == nullptr) {
     return OPERATOR_CANCELLED;
   }