Improve API.md

Author: ehmicky

docs/API.md

@@ -95,7 +95,8 @@ Override the default message generation. Arguments are [`level`](#level),
 _Type_: `boolean`<br>
 _Default_: `true` if the output is a terminal.
 
-Colorize the default [`options.message`](#message).
+Colorize the [`message`](#message). Does not do anything if the
+[`message`](#message) option is defined.
 
 #### exitOn