Updated Design: Persistable Context (markdown)

Design:-Persistable-Context.md

@@ -136,6 +136,14 @@ The table below shows the settting of `settings.js` for each case above.
 | 3    | Exists    | Not exist | Not exist | Throw an error |
 | 4    | Not exist | N/A       | N/A       | Use `memory` module |
 
+### Special case
+* **$file**  
+If a dot is not specified, it is treated as a normal key without persistable context.
+* **$file.**  
+If a key is omitted, throw an error.
+* **$_.key**  
+"_" is a reserved name. It is treated as a memory context.
+
 ## Executing plugin specific function  
 Currently, `get` / `set` / `keys` is only methods to handle Context.  
 In Redis and others, an atomic API such as `INCR` can be used, but it can not be used in the current.