Several textual and markup improvements to the Migrate Upgrade UI

Problem/Motivation

Follow-up to #2905222: Migrate UI: Initial Screen UX improvements. @xjm made some review comments that will be addressed here:

1. +++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
   @@ -142,16 +142,26 @@ public function buildOverviewForm(array $form, FormStateInterface $form_state) {
   -        '#markup' => '<p>' . $this->t('Upgrade a site by importing it into a clean and empty new install of Drupal 8. You will lose any existing configuration once you import your site into it. See the <a href=":url">online documentation for Drupal site upgrades</a> for more detailed information.', [
   +        '#markup' => '<p>' . $this->t('Upgrade a site by importing its database and files into a clean and empty new install of Drupal 8. See the <a href=":url">Drupal site upgrades handbook</a> for more information.', [

   We're not importing the site's database and files. We're importing the data from its database and files.

2. +++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
   @@ -142,16 +142,26 @@ public function buildOverviewForm(array $form, FormStateInterface $form_state) {
   +      $legend[] = $this->t('<em>Old site:</em> the site you want to upgrade.');
   +      $legend[] = $this->t('<em>New site:</em> this empty Drupal 8 installation you will import the old site to.');
   +
   +      $form['legend'] = [
   +        '#theme' => 'item_list',
   +        '#title' => $this->t('Definitions'),
   +        '#list_type' => 'ul',
   +        '#items' => $legend,
   +      ];

   If it's a list of definitions, should it use a definition list?

3. +++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
   @@ -142,16 +142,26 @@ public function buildOverviewForm(array $form, FormStateInterface $form_state) {
   +      $info[] = $this->t('You may need multiple tries for a successful upgrade so <strong>backup the database</strong> for this new site. The upgrade will change it and you may want to revert to its initial state.');

   "Back up" needs to be two words when it's a verb. (It's correct in HEAD.)

   Also "you may need multiple tries" is kind of confusing ("a try" as a noun might not be a familiar expression); "attempts" might be better. But reading this I might wonder why I might need multiple attempts; it's not like shooting a basketball.

   Finally, we aren't using the defined term "new site" here, and if I'm starting from a "clean and empty new install" then why would I want to back up the database of that?

4. +++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
   @@ -160,12 +170,13 @@ public function buildOverviewForm(array $form, FormStateInterface $form_state) {
   +        '#title' => $this->t('Steps to prepare for the upgrade'),

   How about "Preparation steps"?

Proposed resolution

Address comments.

Remaining tasks

Make changes.

User interface changes

API changes

Data model changes