[ticket/11786] Fix typo and ugly English in the notification system PHPDoc

The notification system contained two PHPDoc lines that needed improvement.

PHPBB3-11786
[ticket/11786] Fix PHPDoc documentation of a few events

Fixed some PHPDoc errors I noticed while working on the karma extension.
Mostly spacing issues or omitted characters, though there was one description
that made no sense at all.

PHPBB3-11786
[ticket/11786] s/@return void/@return null in content_visibility.php

The file phpbb/content_visibility.php still contained a few instances of
'@return void' in its PHPDoc blocks, fixed. Thanks to JoshyPHP for reporting.

PHPBB3-11786
[ticket/11786] Further improve wording in phpbb/notification/type/quote.php

Further improve wording in phpbb/notification/type/quote.php per suggestion of
imkingdavid.

PHPBB3-11786
Merge branch 'develop' of git://github.com/phpbb/phpbb3 into ticket/11786
[ticket/11786] Fix indentation in the PHPDoc of core.group_delete_user_before

The core.group_delete_user_before event had bad indentation in its PHPDoc.
Fixed.

PHPBB3-11786
Merge pull request #1654 from rechosen/ticket/11786

[ticket/11786] Fix various defects in PHPDoc in-code documentation