clarify the role of StopIteration

iterators.html

@@ -183,7 +183,7 @@ All three of these class methods, <code>__init__</code>, <code>__iter__</code>,
 <li>&#8220;Calling&#8221; <code>Fib(max)</code> is really creating an instance of this class and calling its <code>__init__()</code> method with <var>max</var>. The <code>__init__()</code> method saves the maximum value as an instance variable so other methods can refer to it later.
 <li>The <code>__iter__()</code> method is called whenever someone calls <code>iter(fib)</code>. (As you&#8217;ll see in a minute, a <code>for</code> loop will call this automatically, but you can also call it yourself manually.) After performing beginning-of-iteration initialization (in this case, resetting <code>self.a</code> and <code>self.b</code>, our two counters), the <code>__iter__()</code> method can return any object that implements a <code>__next__()</code> method. In this case (and in most cases), <code>__iter__()</code> simply returns <var>self</var>, since this class implements its own <code>__next__()</code> method.
 <li>The <code>__next__()</code> method is called whenever someone calls <code>next()</code> on an iterator of an instance of a class. That will make more sense in a minute.
-<li>When the <code>__next__()</code> method raises a <code>StopIteration</code> exception, this signals to the caller that the iteration is over; no more values are available. If the caller is a <code>for</code> loop, it will notice this <code>StopIteration</code> exception and gracefully exit the loop. (In other words, it will swallow the exception.) This little bit of magic is actually the key to using iterators in <code>for</code> loops.
+<li>When the <code>__next__()</code> method raises a <code>StopIteration</code> exception, this signals to the caller that the iteration is exhausted. Unlike most exceptions, this is not an error; it&#8217;s a normal condition that just means that the iterator has no more values to generate. If the caller is a <code>for</code> loop, it will notice this <code>StopIteration</code> exception and gracefully exit the loop. (In other words, it will swallow the exception.) This little bit of magic is actually the key to using iterators in <code>for</code> loops.
 <li>To spit out the next value, an iterator&#8217;s <code>__next__()</code> method simply <code>return</code>s the value. Do not use <code>yield</code> here; that&#8217;s a bit of syntactic sugar that only applies when you&#8217;re using generators. Here you&#8217;re creating your own iterator from scratch; use <code>return</code> instead.
 </ol>