Update description inline when editing iteration

Previously this required page refresh as we weren't
getting descriptionHtml in the update query response

ee/app/assets/javascripts/iterations/queries/create_iteration.mutation.graphql

@@ -3,6 +3,7 @@ mutation createIteration($input: CreateIterationInput!) {
     iteration {
       title
       description
+      descriptionHtml
       startDate
       dueDate
       webUrl

ee/app/assets/javascripts/iterations/queries/update_iteration.mutation.graphql

@@ -4,6 +4,7 @@ mutation updateIteration($input: UpdateIterationInput!) {
       id
       title
       description
+      descriptionHtml
       startDate
       dueDate
     }

ee/changelogs/unreleased/psi-iteration-edit-update.yml

+---
+title: Update description when editing iteration
+merge_request: 40476
+author:
+type: fixed

ee/spec/features/groups/iterations/user_edits_iteration_spec.rb

@@ -46,6 +46,7 @@ RSpec.describe 'User views iteration' do
 
           aggregate_failures do
             expect(page).to have_content(updated_title)
+            expect(page).to have_content(updated_desc)
             expect(page).to have_content(updated_start_date.strftime('%b %-d, %Y'))
             expect(page).to have_content(updated_due_date.strftime('%b %-d, %Y'))
             expect(page).to have_current_path(group_iteration_path(group, iteration))