Add a note about a test client session property gotcha

[Chris Beaven]

I've tripped over this just now, and remember having the issue a while ago too.

A new session object is retrieved every time the client.session property is called
This means the following code will fail:

self.client.session['somekey'] = something
self.client.session.save()

A note in documentation would be useful.

[Luke Plant]

(In [13685]) Fixed #13754 - Add a note about a test client session property gotcha

Thanks SmileyChris for report and patch.

[Luke Plant]

(In [13686]) [1.2.X] Fixed #13754 - Add a note about a test client session property gotcha

Thanks SmileyChris for report and patch.

Backport of [13685] from trunk.