[ci skip] Update config/migration.php comments

Close #3951
diff --git a/application/config/migration.php b/application/config/migration.php
index 083bf28..4b585a6 100644
--- a/application/config/migration.php
+++ b/application/config/migration.php
@@ -21,12 +21,12 @@
 | Migration file names may be based on a sequential identifier or on
 | a timestamp. Options are:
 |
-|   'sequential' = Default migration naming (001_add_blog.php)
+|   'sequential' = Sequential migration naming (001_add_blog.php)
 |   'timestamp'  = Timestamp migration naming (20121031104401_add_blog.php)
 |                  Use timestamp format YYYYMMDDHHIISS.
 |
-| If this configuration value is missing the Migration library defaults
-| to 'sequential' for backward compatibility.
+| Note: If this configuration value is missing the Migration library
+|       defaults to 'sequential' for backward compatibility with CI2.
 |
 */
 $config['migration_type'] = 'timestamp';