info about added global abook and some comments in abook_local_file.php

ChangeLog

@@ -144,6 +144,8 @@ Version 1.5.1 -- CVS
     by Felix Egli.
   - Removed command line option unsupported by qmail-inject in 
     class/deliver/Deliver_SendMail.class.php. Thanks to Ken Brush.
+  - Global file based address book is controled in configuration. Removed
+    global_file address book backend (use 'local_file' instead). 
 
 Version 1.5.0
 --------------------

functions/abook_local_file.php

@@ -21,7 +21,11 @@
  *   filename  => path to addressbook file
  * ? create    => if true: file is created if it does not exist.
  * ? umask     => umask set before opening file.
- * ? name      => name of address book
+ * ? name      => name of address book.
+ * ? detect_writeable => detect address book access permissions by 
+ *                checking file permissions.
+ * ? writeable => allow writing into address book. Used only when
+ *                detect_writeable is set to false.
  *</pre>
  * NOTE. This class should not be used directly. Use the
  *       "AddressBook" class instead.