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.