MDL-40627 add 2G extramemorylimit option to admin UI and use the same units in config-dist.php to prevent confusion

Credit goes to Julien Boulen, thanks for spotting this.

admin/settings/server.php

@@ -182,7 +182,8 @@ $temp->add(new admin_setting_configselect('extramemorylimit', new lang_string('e
                                                  '128M' => '128M',
                                                  '256M' => '256M',
                                                  '512M' => '512M',
-                                                 '1024M' => '1024M'
+                                                 '1024M' => '1024M',
+                                                 '2048M' => '2048M',
                                              )));
 $temp->add(new admin_setting_configtext('curlcache', new lang_string('curlcache', 'admin'),
                                         new lang_string('configcurlcache', 'admin'), 120, PARAM_INT));

config-dist.php

@@ -392,7 +392,7 @@ $CFG->admin = 'admin';
 // memory limit to something higher.
 // The value for the settings should be a valid PHP memory value. e.g. 512M, 1G
 //
-//     $CFG->extramemorylimit = '1G';
+//     $CFG->extramemorylimit = '1024M';
 //
 // Moodle 2.4 introduced a new cache API.
 // The cache API stores a configuration file within the Moodle data directory and