doc: clarify diagnostic behavior in API docs

Updated the description of the  parameter to note
that color output is specific to the spec reporter. This
helps users understand its behavior and create custom
reporters with accurate expectations.

Fixes: nodejs#55922

doc/api/test.md

@@ -2944,7 +2944,7 @@ defined. The corresponding declaration ordered event is `'test:start'`.
     `undefined` if the test was run through the REPL.
   * `message` {string} The diagnostic message.
   * `nesting` {number} The nesting level of the test.
-  * `level` {string} The severity level of the diagnostic message, which determines its output color in the reporter.
+  * `level` {string} The severity level of the diagnostic message.
     Possible values are:
     * `'info'`: Informational messages.
     * `'warn'`: Warnings.