Added note to docs/model-api.txt that EmailField has automatic maxlength=75

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3910 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/model-api.txt

@@ -188,7 +188,8 @@ JavaScript shortcuts.
 ~~~~~~~~~~~~~~
 
 A ``CharField`` that checks that the value is a valid e-mail address.
-This doesn't accept ``maxlength``.
+This doesn't accept ``maxlength``; its ``maxlength`` is automatically set to
+75.
 
 ``FileField``
 ~~~~~~~~~~~~~