document that the string argument in t() should be a literal string

My code reviewer correctly rejected this code:
$msg = 'The username %user is not valid.';
$msg = t($msg, array('%user' => $name));

He noted that:
"The first argument of t() must be a literal string; differently, the script that extracts the string to translate to create the translation template will not be able to extract the string, which would not be translatable"

Indeed that's right, but I didn't find it in the documentation of the t() function for either drupal 6 or drupal 7, so I'm requesting that it's added in the function's comment:

http://api.drupal.org/api/function/t/6