Change sniff rule for @todo to be capitalized and sniff non-php files

For info, there are two open issues in Coder which were added as follow-ups after the original sniff was written.
📌 @todo should start with capitalized letter or non-alpha symbol Active
📌 Create sniff for checking the indentation of lines that follow an @todo comment Active

Completed the 'current documentation'. And changed title because this project does not change sniffs.