[ticket/12723] Use core suffix in filename instead of phpbb in the middle.

PHPBB3-12723
[ticket/12723] Properly describe new Code Sniffer rulsets.

PHPBB3-12723
[ticket/12757] Add a Code Sniffer ruleset for PHP files of phpBB extensions

PHPBB3-12757
[ticket/12723] Do not reference the coding guidelines section.

PHPBB3-12723
[ticket/12723] Do not mention too many details on documentation.

PHPBB3-12723
Merge pull request #1 from bantu/ticket/12723

Ticket/12723