Document "to_array(null)"

documents existing behaviour of the to_array() function when applied to a null value,

Thus:
`search( to_array(`null`), {} ) -> [ null ]`

Author: springcomp

functions/to_array.yml

@@ -11,8 +11,7 @@ returns:
   desc: ''
 desc: |
   * array - Returns the passed in value.
-
-  * number/string/object/boolean - Returns a one element array containing
+  * number/string/object/boolean/null - Returns a one element array containing
   the passed in argument.
 examples:
   test0: