Ensure parent used in the shared examples is not used elsewhere

This is now part of the usage instructions in the relative positioning
shared examples.

spec/models/design_management/design_spec.rb

@@ -12,8 +12,10 @@ RSpec.describe DesignManagement::Design do
   let_it_be(:deleted_design) { create(:design, :with_versions, deleted: true) }
 
   it_behaves_like 'a class that supports relative positioning' do
+    let_it_be(:relative_parent) { create(:issue) }
+
     let(:factory) { :design }
-    let(:default_params) { { issue: issue } }
+    let(:default_params) { { issue: relative_parent } }
   end
 
   describe 'relations' do

spec/support/shared_examples/models/relative_positioning_shared_examples.rb

 # frozen_string_literal: true
 
+# Notes for implementing classes:
+#
+# The following let bindings should be defined:
+# - `factory`: A symbol naming a factory to use to create items
+# - `default_params`: A HashMap of factory parameters to pass to the factory.
+#
+# The `default_params` should include the relative parent, so that any item
+# created with these parameters passed to the `factory` will be considered in
+# the same set of items relative to each other.
+#
+# For the purposes of efficiency, it is a good idea to bind the parent in
+# `let_it_be`, so that it is re-used across examples, but be careful that it
+# does not have any other children - it should only be used within this set of
+# shared examples.
 RSpec.shared_examples 'a class that supports relative positioning' do
   let(:item1) { create_item }
   let(:item2) { create_item }