Updating documentation about parser. Fixes #996

Author: davidkpiano

docs/api/Control.md

@@ -224,9 +224,8 @@ For example, this control validates that a username exists and is longer than 4
 ```
 
 ## `parser={() => ...}` {#prop-parser}
-_(Function)_: A function that _parses_ the view value of the control before it is changed. It takes in two arguments:
+_(Function)_: A function that _parses_ the view value of the control before it is changed. It takes in one argument:
 - `value` - the view value that represents the _next_ model value
-- `previous` (optional) - the current model value _before_ it is changed
 
 **Example**
 ```jsx