Fixed #23639 -- Fixed doc error in RegexValidator.regex

Thanks to @claudep for the report and the original patch.
Fixed #23642 -- Made LocMemCache.incr() thread-safe as documented
Fixed #23063 -- Convert \n and \r to \r\n when using the SMTP backend as per RFC.
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.
Fixed #23468 -- Checks for settings.FIXTURE_DIRS

If there are duplications or default fixture paths
for applications (app/fixture) in settings.FIXTURE_DIRS,
checks will return Warning