Added docs about the new censor function. (#553)

* Added docs about the new censor function.

* Update docs/api.md

Co-Authored-By: mcollina <matteo.collina@gmail.com>

* Update docs/api.md

Co-Authored-By: mcollina <matteo.collina@gmail.com>

docs/api.md

@@ -105,7 +105,10 @@ Each path must be a string using a syntax which corresponds to JavaScript dot an
 
 If an object is supplied, three options can be specified:
   * `paths` (array): Required. An array of paths. See [redaction - Path Syntax ⇗](/docs/redaction.md#paths) for specifics.
-  * `censor` (String): Optional. A value to overwrite key which are to be redacted. Default: `'[Redacted]'`
+  * `censor` (String|Function|Undefined): Optional. When supplied as a String the `censor` option will overwrite keys which are to be redacted. When set to `undefined` the the key will be removed entirely from the object.
+    The `censor` option may also be a mapping function. The (synchronous) mapping function is called with the unredacted value. The value returned from the mapping function becomes the applied censor value. Default: `'[Redacted]'`
+    value synchronously.
+    Default: `'[Redacted]'`
   * `remove` (Boolean): Optional. Instead of censoring the value, remove both the key and the value. Default: `false`
 
 **WARNING**: Never allow user input to define redacted paths.