diff --git a/docs/ref/contrib/admin/index.txt b/docs/ref/contrib/admin/index.txt
index ee1342b43d..29ee66bccc 100644
--- a/docs/ref/contrib/admin/index.txt
+++ b/docs/ref/contrib/admin/index.txt
@@ -816,9 +816,11 @@ subclass::
 
     By default the admin shows all fields as editable. Any fields in this
     option (which should be a ``list`` or ``tuple``) will display its data
-    as-is and non-editable. Note that when specifying :attr:`ModelAdmin.fields`
-    or :attr:`ModelAdmin.fieldsets` the read-only fields must be present to be
-    shown (they are ignored otherwise).
+    as-is and non-editable; they are also excluded from the
+    :class:`~django.forms.ModelForm` used for creating and editing. Note that
+    when specifying :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets`
+    the read-only fields must be present to be shown (they are ignored
+    otherwise).
 
     If ``readonly_fields`` is used without defining explicit ordering through
     :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` they will be