Fixed #21858 -- Clarified 1.6 release note regarding M2M help text changes.

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HTML rendering of model form fields corresponding to

:class:`~django.db.models.ManyToManyField` ORM model fields used to get the

hard-coded sentence

:class:`~django.db.models.ManyToManyField` model fields used to get the

hard-coded sentence:

  *Hold down "Control", or "Command" on a Mac, to select more than one.*

(or its translation to the active locale) imposed as the help legend shown along

them if neither :attr:`model <django.db.models.Field.help_text>` nor :attr:`form

<django.forms.Field.help_text>` ``help_text`` attribute was specified by the

user (or appended to, if ``help_text`` was provided.)

<django.forms.Field.help_text>` ``help_text`` attributes were specified by the

user (or this string was appended to any ``help_text`` that was provided).

This happened always, possibly even with form fields implementing user

interactions that don't involve a keyboard and/or a mouse and was handled at the

model field layer.

Since this happened at the model layer, there was no way to prevent the text

from appearing in cases where it wasn't applicable such as form fields that

implement user interactions that don't involve a keyboard and/or a mouse.

Starting with Django 1.6 this doesn't happen anymore.

Starting with Django 1.6, as an ad-hoc temporary backward-compatibility

provision, the logic to add the "Hold down..." sentence has been moved to the

model form field layer and modified to add the text only when the associated

widget is :class:`~django.forms.SelectMultiple` or selected subclasses.

The change can affect you in a backward incompatible way if you employ custom

model form fields and/or widgets for ``ManyToManyField`` model fields whose UIs

and :doc:`widgets </ref/forms/widgets>` aren't affected but need to be aware of

what's described in :ref:`m2m-help_text-deprecation` below.

This is because, as an ad-hoc temporary backward-compatibility provision, the

described non-standard behavior has been preserved but moved to the model form

field layer and occurs only when the associated widget is

:class:`~django.forms.SelectMultiple` or selected subclasses.

QuerySet iteration

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