Cleaned up the Memcached cache backend doc

Consistent casing, minor typo fixes, reduced rhetoric.
Fixed #23638 -- Prevented crash while parsing invalid cookie content

Thanks Philip Gatt for the report and Tim Graham for the review.
Fixed #23534 -- Added documentation on blocktrans tags

Mentioned that other block tags are not allowed inside
a blocktrans template tag.

Thanks to edu2004eu for reporting the issue.
Adapted invalid cookie test to all Python versions

Refs #23638. Older Python versions are less strict when parsing
invalid cookie content. The test just has to ensure Django doesn't
crash.
Clarified documentation relating to usage of serialized_rollback for certain classes
Fixed #23651 -- Isolated non-existent fixture tests

Previous versions of the tests were buggy, as initial_data.json
did exist and the test wasn't failing. It was finally failing on
Python 3.4.2.
Thanks Raphaël Hertzog for the report (and Debian bug #765117
contributors).