Change the description for username field in user registration
Another couple of options using string translation techniques:


Contrib module: String Overrides
settings.php snippet insert


The string overrides module lets you do the string translation via the UI.
Otherwise, you can set the override in settings.php - the search string needs to match exactly.

/**
 * String overrides:
 *
 * To override specific strings on your site with or without enabling locale
 * module, add an entry to this list. This functionality allows you to change
 * a small number of your site's default English language interface strings.
 */
$conf['locale_custom_strings_en'][''] = array(
  'Spaces are allowed; punctuation is not allowed except for periods, hyphens, apostrophes, and underscores.' =&gt; 'Spaces and punctuation are not allowed except for periods, hyphens, apostrophes, and underscores.',
);