diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 91ca349..abc2816 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1515,21 +1515,33 @@ function t($string, array $args = array(), array $options = array()) {
 }
 
 /**
- * Replaces placeholders with sanitized values in a string.
+ * Formats a string for HTML display by replacing variable placeholders.
+ *
+ * This function replaces variable placeholders in a string with the requested
+ * values and escapes the values so they can be safely displayed as HTML. It
+ * should be used on any unknown text that is intended to be printed to an HTML
+ * page (especially text that may have come from untrusted users, since in that
+ * case it prevents cross-site scripting and other security problems).
+ *
+ * In most cases, you should use t() rather than this function, since it will
+ * translate the text (on non-English-only sites) in addition to formatting it.
  *
  * @param $string
  *   A string containing placeholders.
  * @param $args
  *   An associative array of replacements to make. Occurrences in $string of
- *   any key in $args are replaced with the corresponding value, after
- *   sanitization. The sanitization function depends on the first character of
- *   the key:
- *   - !variable: Inserted as is. Use this for text that has already been
- *     sanitized.
- *   - @variable: Escaped to HTML using check_plain(). Use this for anything
- *     displayed on a page on the site.
- *   - %variable: Escaped as a placeholder for user-submitted content using
- *     drupal_placeholder(), which shows up as <em>emphasized</em> text.
+ *   any key in $args are replaced with the corresponding value, after optional
+ *   sanitization and formatting. The type of sanitization and formatting
+ *   depends on the first character of the key:
+ *   - @variable: Escaped to HTML using check_plain(). Use this as the default
+ *     choice for anything displayed on a page on the site.
+ *   - %variable: Escaped to HTML and formatted using drupal_placeholder(),
+ *     which makes it display as <em>emphasized</em> text.
+ *   - !variable: Inserted as is, with no sanitization or formatting. Only use
+ *     this for text that has already been prepared for HTML display (for
+ *     example, user-supplied text that has already been run through
+ *     check_plain() previously, or is expected to contain some limited HTML
+ *     tags and has already been run through filter_xss() previously).
  *
  * @see t()
  * @ingroup sanitization