+ * Get the default (plain) message contents for an page that overrides an
+ * interface message key.
+ *
+ * Primary use cases:
+ *
+ * - Article:
+ * - Show default when viewing the page. The Article::getSubstituteContent
+ * method displays the default message content, instead of the
+ * 'noarticletext' placeholder message normally used.
+ *
+ * - EditPage:
+ * - Title of edit page. When creating an interface message override,
+ * the editor is told they are "Editing the page", instead of
+ * "Creating the page". (EditPage::setHeaders)
+ * - Edit notice. The 'translateinterface' edit notice is shown when creating
+ * or editing a an interface message override. (EditPage::showIntro)
+ * - Opening the editor. The contents of the localisation message are used
+ * as contents of the editor when creating a new page in the MediaWiki
+ * namespace. This simplifies the process for editors when "changing"
+ * an interface message by creating an override. (EditPage::getContentObject)
+ * - Showing a diff. The left-hand side of a diff when an editor is
+ * previewing their changes before saving the creation of a page in the
+ * MediaWiki namespace. (EditPage::showDiff)
+ * - Disallowing a save. When attempting to create a a MediaWiki-namespace
+ * page with the proposed content matching the interface message default,
+ * the save is rejected, the same way we disallow blank pages from being
+ * created. (EditPage::internalAttemptSave)
+ *
+ * - ApiEditPage:
+ * - Default content, when using the 'prepend' or 'append' feature.
+ *
+ * - SkinTemplate:
+ * - Label the create action as "Edit", if the page can be an override.