Fixed #34595 -- Doc'd that format_string arg of format_html() is not escaped.

docs/ref/utils.txt

@@ -612,8 +612,10 @@ escaping HTML.
 .. function:: format_html(format_string, *args, **kwargs)
 
     This is similar to :meth:`str.format`, except that it is appropriate for
-    building up HTML fragments. All args and kwargs are passed through
+    building up HTML fragments. The first argument ``format_string`` is not
+    escaped but all other args and kwargs are passed through
     :func:`conditional_escape` before being passed to ``str.format()``.
+    Finally, the output has :func:`~django.utils.safestring.mark_safe` applied.
 
     For the case of building up small HTML fragments, this function is to be
     preferred over string interpolation using ``%`` or ``str.format()``