Document using commit=False on forms.

docs/forms.txt

@@ -33,3 +33,19 @@ apple "ball cat" dog   ``["apple", "ball cat", "dog"]``  No commas, so space del
 "apple" "ball dog      ``["apple", "ball", "dog"]``      Unclosed double quote is ignored
 ====================== ================================= ================================================
 
+
+``commit=False``
+~~~~~~~~~~~~~~~~
+
+If, when saving a form you use the ``commit=False`` option you'll need to call
+``save_m2m()`` on the form after you save the object, just as you would for a
+form with normal many to many fields on it::
+    
+    if request.method == "POST":
+        form = MyFormClass(requets.POST)
+        if form.is_valid():
+            obj = form.save(commit=False)
+            obj.user = request.user
+            obj.save()
+            # Without this next line the tags won't be saved.
+            form.save_m2m()