Docs: Standardize on 'backward compatibility/compatible' nomenclature in core inline docs.

Also use 'back-compat' in some inline comments where backward compatibility is the subject and shorthand feels more natural.

Note: 'backwards compatibility/compatibile' can also be considered correct, though it's primary seen in regular use in British English.

Props ocean90.
Fixes #36835.

Built from https://develop.svn.wordpress.org/trunk@37431


git-svn-id: http://core.svn.wordpress.org/trunk@37397 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/options-head.php

@@ -11,7 +11,7 @@
 wp_reset_vars( array( 'action' ) );
 
 if ( isset( $_GET['updated'] ) && isset( $_GET['page'] ) ) {
-	// For backwards compat with plugins that don't use the Settings API and just set updated=1 in the redirect
+	// For back-compat with plugins that don't use the Settings API and just set updated=1 in the redirect.
 	add_settings_error('general', 'settings_updated', __('Settings saved.'), 'updated');
 }