Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Tweaked unit test 'quick start' explanation.

Thanks Jeremy Dunck.
Fixed #20471 - Typos in topics/forms/formsets.txt
Fixed #9321 -- Deprecated hard-coding of help text in model ManyToManyField fields.

This is backward incompatible for custom form field/widgets that rely
on the hard-coded 'Hold down "Control", or "Command" on a Mac, to select
more than one.' sentence.

Application that use standard model form fields and widgets aren't
affected but need to start handling these help texts by themselves
before Django 1.8.

For more details, see the related release notes and deprecation timeline
sections added with this commit.
Removed a confusing duplicate SESSION_COOKIE_DOMAIN header

The note is clearly a part of MESSAGE_STORAGE documentation. As a separate
section, it broke automatic link generation on the HTML version of the
documentation.
Fixed #20472: response.content should be bytes on both Python 2 and 3
Fixed #20212 - __reduce__ should only be defined for Py3+.
Merge pull request #1191 from ambv/content_is_bytes

Fixed #20472: response.content should be bytes on both Python 2 and 3
Merge pull request #1190 from ambv/doc_settings_fix

Removed a confusing duplicate SESSION_COOKIE_DOMAIN header
Use assertIsInstance in tests.

Gives much nicer errors when it fails.
Merge pull request #1192 from mjtamlyn/assertIsInstance

Use assertIsInstance in test code.
Fixed test failures from 09f865276554f35060ff939722ec4cefd578edf6.
Fixed a Python 2.6 regression (GzipFile can't act as a context manager)
Fixed #17308 -- Enabled the use of short_description on properties in the admin.
Merge pull request #1194 from ambv/python26_is_fun

Fixed a Python 2.6 regression (GzipFile can't act as a context manager)
Imported copyreg from six.moves
Fixed #6412 -- More details if a template file cannot be loaded

Report more details about template files in loader postmortem.
(Re-)moved some imports
Fixed #19326 -- Added first() and last() methods to QuerySet
Slightly reworded 'last()' docs.