[ci skip] Improve note about sess_save_path in config.php
diff --git a/application/config/config.php b/application/config/config.php
index 7640df4..1e39959 100644
--- a/application/config/config.php
+++ b/application/config/config.php
@@ -328,12 +328,14 @@
 |
 | 'sess_save_path'
 |
-|	The location to save sessions to, driver dependant
+|	The location to save sessions to, driver dependant.
 |
-|	For the 'files' driver, it's a path to a directory.
+|	For the 'files' driver, it's a path to a writable directory.
 |	For the 'database' driver, it's a table name.
 |	Please read up the manual for the format with other session drivers.
 |
+|	IMPORTANT: You are REQUIRED to set a valid save path!
+|
 | 'sess_match_ip'
 |
 |	Whether to match the user's IP address when reading the session data.